Gas milage for a diesel / gas engine

Here's my request, I need people that own fifth wheel trailers to let me know about their gas mileage. I presently have a toyota tundra when I'm not towing anything I get about 20 miles to the gallon but when I'm towing my TT the gas mileage drops to 9 miles to the gallon. NOW WE ALL KNOW that truck sales men will tell you that their truck gets a million miles to the gallon which we know isn't true. So if you have a fifth wheel trailer please tell me which truck you tow with and is it gas or diesel and what your gas mileage is. This will help my wife and I to pick the correct truck and fifth wheel to buy.