and I'd spend money first on a good used truck.
I'd jump to a dually unless you have daily driver or parking issues.
Gas is okay if you'll never tow with the camper on.
Diesel is usually better if hauling a heavier camper and/or towing.
Mileage v costs v maintance/repairs of the gas v diesel is always hated algebra.
A sweet spot would be a 2002-2006 Dodge Quad Cab 5.9 Cummins 3500 or any of the Ford 7.3 Powerstrokes. (I don't know anything about newer GM trucks)
I believe the Ford V10 to be a pretty good motor for hauling a camper.