I have a RAM 2500 gasoline engine TV. It's been a great truck. Plenty of power for mountains, acceleration to get onto any freeway while pulling the TT, easy to find fuel.
But it is a bit more thirsty than a diesel.
If I want to go to a bigger, heavier trailer than 36 ft / 10,000 lbs fully loaded - I need diesel.
Diesel slightly better fuel mileage will NEVER pay the cost difference between purchase of a gas engine and a diesel engine vehicle. Nor will maintenance savings make up the cost difference.
But after 26,000 miles towing my trailers, averaging 24.809 cents per mile for fuel - I will tell you that diesel is the way to go if you can afford the $8-11,000 higher purchase price for the diesel engine.
Buy the most truck with the highest towing rating you can get. Also, very, very important - check the door stickers and compare the cargo capacity number between different vehicles on the dealer's lot. I've found some recent RAM 3500's with 1,000 lbs less cargo capacity for a fully loaded model vs a simplier model.
That translates into 1,000 lbs less hitch weight. It might be the difference between the fifth wheel you want, and having to settle for less.