The debate of "gas vs diesel" is ages old and never really changes.
Yes, the diesel will give you the best overall performance, the best overall fuel economy, and will put a smile on your face every time you press the skinny pedal, IFFFFFF you can get past the co$t aspect of it, and realize that it will NEVER pencil out financially.
A diesel is going to cost more, period. More to own. More to operate. More to maintain. That is a fact. You will not come out ahead financially owning a diesel for towing a camper 5000 miles a year. You will, however, get to the campground faster.
There is no reason a gas engined truck can't tow a 5th wheel. Asking such is really silly. You just have to get past the fact that the engine is going to run at more then 2000RPM, probably for long stretches. You are NOT hurting the engine. You are NOT hurting the truck. Gasoline engines need to rev to make power.
There are also different levels of "5th wheel." You can get anything from a 7000lb up to a 21000lb triaxle behemoth. You probably want something more than a 3/4 ton gasoline engined truck for the larger trailers.
Ride quality difference would be negligible between the trucks. I've never heard anyone even try to create a rumor to that effect. If anything the diesel should ride better due to the extra weight.
Putting 10-ply tires on half ton trucks since aught-four.