I regularly tow my 20' enclosed with my 4.6l 2002 F150 with a tow package and 3.55 gears. My truck maintains speeds up to 61-62 mph on flatland pretty effortlessly (2200-2700 rpm). That seems to be the sweet spot. It can struggle to maintain 65-70 mph. It depends on the conditions (wind, hills...). I don't try to hold those speeds unless I'm really anxious to get home after a long weekend and the conditions allow it. I have one small pass that I go over to get to Laguna Seca (Pacheco Pass). It can do it at 60, but that's at 4000-4500 rpms. I just keep it to about 3000 rpms and I can go about 50 mph. Fuel mileage sucks and it doesn't matter how I drive. It's 9-10 mpg while towing.

I have the stabilizer bars for the trailer but I don't have the right hitch to use them. After my first couple of tows, I decided I didn't need anything else. The whole setup is very stable and pulls nicely.

Having said all that, yes, a diesel, or a big block and a one ton might be the ideal setup, but I'm pretty content with how my truck does the job. I am positive that you will be happy with the job your 5.4l Expedition does. (I do not know anything about the suspension though). Happy towing.