I wonder if you did the resistance test and walked around banging on the wheels while doing that, see if values change. The other thought would be the type of brakes. It still could be an issue in the plug on the trailer, would not take much for some corrosion/loose connection to keep a good tight connection. From what I read, the truck is putting a pulse of power into the brake wire to see if something is there. I'm sure it would work fine if it was powered all the time, but then the brakes would be applied.

 

My story on this:

Brand new truck. Hooked up my fairly new and very nice 10' aluminum flatbed and had all sorts of messages, but some lights worked. Trailer worked just fine on my 01 SCREW. Decided to give my old junk 12' homebuilt a try and it worked just fine. I was figuring all sorts of possibilities like not compatible with my LEDs and such or maybe truck too sensitive to my lights. Using my multi meter no shorts were showing. Didn't do a full test, just the beep for a short function.

Before embarrassing myself and taking a good truck in for warranty work, I decided to take the trailer over to my local trailer supplier and ask if they were aware of any issues common to the F150. They were fairly dismissive and said "Nope, your trailer wiring is bad". I went ahead and let them do their thing and within five minutes they had found were rodents had chewed my harness (inside a box frame rail, not exposed) but not enough for a dead short. A couple hundred bucks and a new harness later all is well.

My 20' enclosed will occasionally show error messages when I'm an hour or so into a trip. Sure enough, every time my truck shows a fault the fault is correct. Problem with my 20' enclosed is that it has a flakey connector. Can't really do much to correct without replacing it.

End result is that I'm really liking the trailer fault detection on the new truck. Was frustrating at first but it has really helped me get my trailer wiring in shape. Also don't bother with light checks anymore because the truck will let me know if a lose a light.

 

Thanks for the replies here and sorry for the delay back. I decided to dig into this more thinking it has to be the trailer with all the reading Ive been doing a lot of reading on this subject. Sure enough i found that 3 of my 4 electronic brakes had a severd wire or completely cut and had no idea where it was connected to. i contacted the manufacture of the trailer and they provided me with some direction on wiring up the brakes again. I went for a 20 min drive and nothing came up. I am hoping it was a fault with my trailer brakes. I will report back if it comes back up. if it does i think im going to replace the electric brakes as they did feel weak still due to the inside of the drum being very rusty. My guess is i drove around with broken wires for a while and its going to take some time for the pads to clean the surface rust off on the inside of the drums. The one wheel that had all its connections locked the tire up complete when manually using the brake controller which i wasn't able to do before. fingers crossed i am hoping it was my trailer.

Notes:
i did use a bunch i dielectric grease on both the front and backside of the trailer plugs on the truck. the plug is brand new.
I cleaned out the 7 way on my trailer even though its always been packed with grease.
used a new hitch that wasn't as sloppy

 

This past Friday my 2018 Lariat just started giving me a trailer disconnected message on screen, I try to use the factory brake controller to manually set the brakes to adjust gain and get the trailer disconnected message. When I hook it up it shows trailer connected but shows Cross Traffic/Bliss alert disabled. When I try to squeeze the manual adjuster I get the trailer disconnected message and the trailer brakes do nothing. Now I'm looking for solutions.

 

^^Update 2018 F-150 Lariat.....crawled under the trailer some rodent species chewed up one wire almost in half and one other was unshielded. So I think I found my problem. I found 2 others when I tore into it. Repaired the wires and all works as advertised, including BLISS. Sure glad I looked before going to the dealership. Maybe not in all cases but certainly mine, if you are getting an error there's probably something actually wrong and not just a ghost warning message.

 

I'm having the exact same issue on my 18, I'll check the wires at the wheels.

 

I have a 15 5.0 and last fall while pulling one of cars in a 22' enclosed on the pa turnpike all of a sudden I get the 'trailer disconnected' followed bu the brake gain display and a ding. I pull over and check everything, and keep going. It did it every 10 mins all the way home. I park the trailer all winter til last week to bring home a 69 Mach 1 SCJ a friend bought and sure enough same thing.. I search the net to find this thread and take the truck to a local shop to see if there were any updates to the trailer brake module and sure enough there were. He up dated it and I tried it today and the issue is gone. Weird. In the middle of towing after years the outdated software causing this. So the first thing is to do this instead of diving into the trailer wiring.

 

SECOND THIS. Take it in for an update. I was having the exact same issues as OP describing with my snowmobile trailer. So annoying going down the highway for several hundred miles. Trailer Connected. Trailer Disconnected. Drives a guy crazy.

Took it to dealer and they found there was an update that fixes the issue. They then tried to charge me $250 for the update. Called BS, asked to speak to higher up, because its a bug in their software from the beginning. Shouldn’t be something put on the consumer. Ended up paying like $110. Not as bad but still. Just a heads up as something you might want to consider.