The F-350 diesel I had previous to the F-250 gasser was purchased for
the same reason ... towing a fifth wheel RV. It delivered above 20
mpg in normal use ... assuming the 6.0L engine it had ran.
I had tried a GM 2500 diesel as well and it rode a lot softer than
the Ford. It was a nice truck but I went for the Ford in the end.
Should have researched the problems with that engine more.
Worst riding truck I had was a 1998 Dodge Ram 2500. Rode like
a brick but was otherwise a nice truck. The Fords ride better
but not as nice as the GM offerings.
I haven't tried a F-150 though. I've heard they ride like a
large car, being on a different suspension than the Superduty
Fords.
The f150 is same as an expedition.
The older ones were built on the same chassis as the Town Car.