real world mileage, diesel vs gas

Passin Thru wrote:
If you are worried about mileage , don't buy a Motorhome. Diesels produce more torque than gas . You can figure out HO produced from a gallon of diesel with this formula .450 to .325 (lbs/bhp/hr.)but you must factor in when you use full HP , 1/2 hp or none, like coasting down the mountain. Figure between 6 and 8.5 MPG for a 350 hp engine depending on tailwind, if you are going east out of Amarillo, west out of OKC or N out of Albuquerque. You mileage may variy. My Peterbilt averaged 6.8 MPG rated at 500 HP for 420,000 miles including mountains plains and idling in hot and cold weather.
SCOFF SCOFF 12MPG Bul)&^%$ Come to VA and hook on my trailer, GWT 10,500 and go from Va to Pittsburg and back and if you get over 10 MPG, I'll buy the fuel and the steak. No one has ever taken me up on that challenge yet

mileage is a concern because I'm trying to decide between a fifth wheel toy hauler which I currently get 12-13 pulling with a diesel vs an RV. I can't imagine a 33-35' diesel rv would be any heavier than a 3/4 ton pickup and a 35' fifth wheel.