I get 12-15 mpg depending on how hilly it is with a 3k lb camper on the back. SRW 1 ton Dodge Ram 3500 4x4, quad cab, diesel, 6 speed.
A gas truck isn't going to do 15 mpg with a heavy camper but the newer trucks with 2 overdrives and 3.73s should have no problem with power or mileage as long as it's a diesel.
A gas engine will be lucky to do 10 mpg with the same load. Even if it is similar in size and weight, a gas engine will need 4.10 or deeper gears and gas engine has less energy per gallon than gasoline and it adds up.