My opinion--your my vary.
I agree with above that diesels will get about 30% better fuel mileage but at a significantly higher initial cost along with generally higher fuel cost. Also, wind resistance is huge. With our Ford 6.7 diesel we got 11 mpg (within .5 mpg) for our square nose 6500 lb bumper pull, a 9400 lb fifth wheel and now a 12700 lb fifth wheel. My only explanation is that each one must have been more aerodynamic than the last one. 7 mpg would be disappointing, but gas engines do suck down the fuel. We got 8.5 gas pulling the bumper pull. Certainly a 250 with any engine would be a better towing experience (the 250 with a 6.2 only comes with a 6 speed) but at a repeat initial cost.) If OP only tows occasionally I would put up with the fuel cost which is probably small compared to the new truck cost right now with limited supply.