Towed a decent weight with my 2018 HDPP/MTTP truck today about a 100 miles on rural highways.
The trailer was a heavy duty flat bed about 28 feet long with a Porsche 924 on it (towing for a friend, wish it was mine) no real idea of total weight but probably around 6000 to 6500 pounds.
The truck squatted very little maybe an inch. The towing power was effortless, the boost gauge did not show any blue for much of the trip. A little vibration at 50 mph, might be road or trailer but smoothed out at 60 mph! Very stable at these speeds, really forgot it was there! This thing engine brakes like crazy, to the point where my wife said "what are doing" and I said nothing it's the truck! The 3.5 does lack a little when engine braking, can see where it might not be as effective in the mountains.
Perhaps the biggest surprise Mpgs! Four passenger a little gear and the trailer 14.2 L/100 (about 16.5 Mpg US)

Over all very pleased.

 

Hmm interesting you say vibration at 50 MPH. Mine does this too, but when empty. Only have 300 miles on, but plan to tow my RV a week from Friday, should have 1300+ miles on by then, and will know if the vibration continues, goes away or gets worse. Mine is a Plat so no HDPP, but is a Max Tow. That 3.5 is a beast and tows like mad up mountains.

 

The truck has no vibration when not towing.. i really think it was the trailer,and road combination the trailer had been sitting for 3 or 4 years in the dirt with car on it, and they were rural Ontario roads! a

 

Very likely if thats the case. I had purchased a new 6x12 flatbed trailer and all seemed fine, until I hit 60 MPH, then the entire truck shook like crazy. The idiot who put the wheels on didn't torque them properly so both wheels were warped. Once I unbolted, rotated them one hole around and torqued them properly the vibration was gone.

 

The manual says some vibration may occur while towing/hauling due to driveshaft/ujoint angle when the rear suspension is compressed. But with only 1" that seems awful early to see it.

 

They are spot on with this. my 2014 would shake from a dead stop with my trailer hooked up, or if I had 1000 pounds in the bed and accelerated hard. Since I had read that part before towing I knew what it was. However only under acceleration was it apparent, under normal cruise it was not. Another week and I can tow my RV and see how it compares to my last truck and the 14, and I have a strong feeling it will handle like the 14 did, which will make me ecstatic. The 18 drives so much like the 14 did and soooooo much better than the Red Devil, it's like the Red Devil wasn't even a Ford.

 

What truck did you have before this one? One of the older steel ones? The ecoboost has never been short on power what it's short on it weight and a heavy enough suspension to handle trailers in the wind.
Hitch up to a high profile trailer on a windy day and take it for a test drive and report back how it felt.
I just bought a '16 SD to do most of my towing because the wind blows me all over the road with this '17 and that's when I am only towing a 1800 lb RZR on a 12' trailer (max tow truck not HDPP) I just can't imagine that the HDPP is going to be much better

 

A picture of the truck and trailer. The car and trailer look very small from the angle. The Porsche is 3200 and the trailer is about the same, the truck looks to squatting far more then it really is.

 

 

My 2 cents...Towed this last weekend with our HDPP 18' truck, tows well and stable but I have added one leaf to each spring pack for a total of four leafs a side and a set of timbrens , This is my 5th HDPP truck since 2012 and we set them up for heavy heavy towing most of time.