I have scoured the Internet as much as I can and have not found this topic covered anywhere, including in this forum, so if I missed a previous discussion on this, forgive me.
I have a 2020 F-150 Lariat 5.0 and tow a 26 travel trailer with a standard lead acid deep cycle on it powering a 12v fridge. We haven't had it long and I've only towed it long-ish distance (250 miles) once. The truck had about 20k miles on it at the time. During that trip, I had the battery disconnect switch turned on to power the 12v fridge, mostly as an experiment to see if the truck would keep the fridge going while cruising down the road. Before the trip, I did all the Internet research about if this was possible and read all the comments about "Run a dedicated wire from the truck battery back to the trailer for full voltage/amperage", which I of course didn't do, and "It should work fine but worst case scenario, the trailer batter dies and the fridge quits.". The result I expected was somewhere between worked flawlessly and the battery on the camper died. What I did not expect was to stop for gas about 100 miles in and have the truck almost not start back up. So of course after that I thought "To hell with the stuff in the fridge" and turned the fridge off and shut off the disconnect switch for the remainder of the trip, assuming the alternator was just not strong enough to get the job done. Over the last several months since then, (during which time I was considering trading in for an F-250 with dual batteries and alternators and then subsequently almost vomiting at the sticker price) I have been hyper-sensitive to paying attention to how the truck cranks up, with it having a slightly slower crank up a few times since then. So now I'm wondering if I don't just have a bad battery. I've had two previous vehicles with factory AGM batteries end up with a "dead cell", according to the dealer, and replace the battery sooner than expected. We've got another trip coming up in a month or so and am considering pulling the trigger on an Odyssey or Optima battery, mostly out of paranoia.

My question for the group is this: Is driving down the road running your travel trailer's 12v fridge standard ops for most of you and I just have a POS battery? Or is the battery fine and I'm just asking too much of it and/or the alternator?

Thanks

 

I'm sure you will get plenty of responses with a ton of good info, but I'd start simple and have the truck battery checked. The original battery in my '19 F350 was changed in '22 after a little over 3 years of service. I live in a hot climate (AZ), so not unusual for a battery to have a 3-4 year lifespan.

After I replaced the battery we switched over from gas absorption to a 12v fridge in our TT. I ran the same setup as you for a couple thousand miles. The F350 was equipped with what is called the 'extra extra heavy duty alternator', so this is not an apples-apples truck comparison. However, from what I was told, one of the challenges to charging via the 7-pin is the small wire gauge, so not certain alternator size has all that much to do with it as the wire can only send so much juice down the line. In the 2k or so towing miles with the fridge running, I had zero issues arriving at my destination with the fridge nice and cold and no truck battery issues. We typically tow 5-7 hours per day when on a trip.

Like you, I received many responses stating a separate charge line and/or a DC-DC charger was the way to go, and also many stating the 7-pin alone would be fine. I'm curious to see what you find to be the issue.

On a side note, we no longer have the trailer above. The new trailer has 400w of solar to augment charging!

 

My understanding and it may be incorrect, is that you are not actually running your fridge off of the truck power but the camper battery. The truck alternator is charging the camper battery. So I guess the truck is indirectly running the fridge but at more of a steady rate just to charge the battery.
In short, I would guess the battery is on its way out.

 

You are 100% right. The fridge is running off of the trailer's deep cycle battery and the 7 pin simply offers a bit of trickle charging (so to speak) for the trailer battery. Yeah, I'm leaning more towards the crappy battery theory. Thanks.

 

Ok, that's two votes for junky battery, not including my own personal bias. I guess that's enough to convince me to spend a bit of cash on a new one. Eventually, like you, I would like to upgrade to a solar charging system of some kind, but so far in 9 months we have only used the camper once (soon to be twice) so spending a bajillion dollars on a solar charging system, or trading in for a camper that already has it, does not fit into my life plans. Lol. Hoping to use the camper a bit more soon though.

Thanks

 

Have the truck battery tested fully.
Strongly consider replacing camper battery with a LiFePO4 ($3-400 range if you avoid the big name, well advertised brands).
Have you noticed the big push of 12VDC refrigerators also has a big push on to include small solar power (100-200 watt panels) systems?
You have experienced the reason why.
Yes, you really should run heavy cable via a large Anderson connector from your truck battery to a DC-DC charger into your RV DC system.
Adding solar should be considered but can be a future project. The good power from the truck to the RV should be done prior. First, a better RV battery, 100AH LiFePO4, will last twice as long powering the fridge as a standard lead acid before it tries to suck all the power it can from your truck to keep the fridge running. Remember, a lead acid battery can only use HALF its rated amperage before battery damage can occur, a LiFEPO4 can be run completely down with little damage. If you get a 2nd one, paralleled, you essentially quadruple your power available at the same weight as the original single lead acid.

 

The alternator is rated at 240amps afaik.

I'd lean towards ensuring the camper's system battery is a) in good condition and B) fully charged before I left. Campers/trailers battery systems are well suited to solar panels.
Trailers also have a battery to activate the emergency brakes which typically discharge after months of inactivity and recharge en route to the destination. .

Guess (y)our pre trip check list just grew by a few items.......

 

I read that whole thing and I don’t follow where there’s a dead battery in there. One weird start wouldn’t convince me anything is going on other than what we already know, and that’s that these newer trucks don’t charge the RV battery, but merely maintain, it while going down the road.

That said, if your truck battery is going dead, something is wrong not related to the design of the truck. The most you can draw through that circuit is 30 A and the F150 has an alternator that can support that easily if it ever reached that point. Which it won’t without a buck boost transformer.

 

I feel like we might be splitting hairs on the definition of "dead battery", or perhaps I didn't get my point across that well. I do tend to ramble sometimes. The "almost dead battery" I eluded to was mentioned during the gas stop on the trip in question. It wasn't just a "weird start". The starter barely even cranked, just like it would have done with, let's say, an almost dead battery. In any case, the reason I have been blaming this event in the camper is because the truck had shown no signs of a battery issue whatsoever before that exact moment, and since then, it has had several (which is more than one) starts where the starter was a bit slow to start with no trailer or anything attached at the time. Some would consider those starts to be "weird". I'll admit again, as I did above, I was hyper-sensitive to how the truck was starting since the infamous moment. All that coupled with the history I've had with modern Ford batteries, that I also mentioned, had me already leaning towards a battery issue. I just mostly was looking for some insight for seasoned travel trailer enthusiasts as to whether or not the battery issue was my fault for expecting too much from the truck's charging system. And I think you guys have sufficiently answered my question. In any case, it was fun mapping out my original comment to make sure I hit all the bullet points. Spoiler alert: I did. I will test the battery and more than likely replace it before the next camping trip just to be safe. Until next, Internet......

 

The wire going through your 7 Pin on the truck side that supplies 12V is only 16 AWG. I measured around 2A going through it charging a lithium battery. With an AGM or other lead acid, you might get a slight bit more. I actually disconnected that wire now since it does so little.

You have bad batteries in both your trailer and your truck.

I tow with a 12V compressor fridge but I have 560AH of lithium battery, 800AH of solar and a 30A DC to DC converter which is only used when I am travelling from one shady campsite to another to get max charge.