I can't speak to other's experience, but my 02 pulling my enclosed will get about 8.5-9mpg... Idle time and larger than stock tires contribute to that also... Summer driving with stock 265s I can get 12.5-13 on the interstate empty or about 11 in town empty driving within 2-4mph of the speed limit. Switching the summer 31s to my winter set of 33s along with warm up time in the winter costs about 1.5-2mpg off of those summer numbers. It really saves me some money to switch back to stock tires in the summer... when you only get around 10 mpg, a 1.5-2 mpg increase is saving you 15-20% on your fuel economy.
The biggest thing I notice when pulling isn't the weight of the enclosed, but just the fact that it catches so much wind. If you hook up and then walk about 100 ft in front of the truck and look back at it, the trailer almost dwarfs the truck being an 8.5 wide and 3-4 ft taller as well... It's like trying to tow a sail behind you.
I take my truck to the dealer for oil changes as they advertise that 'Ford Works' special all the time for a good price and they always tell me how rare it is to see a V-10 in the shop for service... They just don't have the failure rate of the diesels since they're a lot simpler motor (and admittedly don't make as much power). The biggest issue the pre-05 trucks had was ejecting spark plugs, but that was fixed in 05 with the new head design which also bumped up the hp and tq numbers a bit.