I have a 14,000 lb fifth wheel that I tow with a 6.0 Ford Diesel. Mileage depends on the terrain, but normally I average just under 12 mpg. I've gotten as good as 14 mpg and as bad as 9.9 mpg when climbing in the Rockies.
My biggest issue has been tires. The rig has two axles and takes ST235/85R16s, so it's right at the allowable weight for the four E load range tires. I try to keep as little unnecessary weight in the RV as possible. I dump the tanks before starting off in the morning and keep the freshwater tank at about a quarter full. I've blown out four tires on this rig and two of the blowouts did over a thousand dollars in damage.
I check the tire pressure every morning before I start off and I do a walk-around every time I stop; I check for loose fenders, dragging objects, and overheating tires. I drive no more than 65 mph.
I've found a couple of RV parks where I couldn't get in because of the layout of the park. Several of the ones I visited on the Eastern side of the Sierras have a max length of 24 feet.
