I was in the OP's position when we decided to purchase a fifth wheel. I knew which one I wanted and it weighs about 12K. Bought a 2017 Ram 3/4 ton with the 6.4 hemi - payload was awesome, added the pucks, plopped in a B&W Companion and off I went. Reasons for buying the gasser included listening to friends and family saying I didn't need the diesel, being afraid of stuff that could go wrong, expensive maintenance -the whole gamut.
Brought it while still owning the old Coleman and dragged it around from storage to home, and one last trip. Really nice towing. Bought the new fiver and went on our first trip. Truck was very capable, but the constant roaring in the high RPM bands even up overpasses really gets to me - did with my old truck too - thought it would be different - it wasn't.
Fast forward 5 more trips and I was ready to be out of it - looked at all the used apps for the same thing, a few years old truck with a diesel - I'm stuck on the Ram - sorry Fish, plus my dad worked there for 35 years so I got the A plan - which dealers in non automotive producing states really hate.
I ended up seeing my present truck - a 2018 Ram 3500 with the Aisin and CTD at a dealership, got the deal I wanted for the trade, and man o man, it just rolls up those hills barely peaking 2k RPM and it eats bridges for lunch - it just goes. There is truly nothing like it. I took the west belt of 295 coming home from a trip so I could blast up the Dames Point Bridge and use the exhaust brake coming down the other side - with one hand on the wheel.
I stash money in a savings account specifically for maintenance as the oil changes do cost more, but at 22K miles, I'm only on the second one.
And don't go for the 250 if possible - friend bought an Alpine - tows it with a 2017 F25 he got at Auto Nation. He showed me pictures hooked up and was distressed on how much the rear sagged - so he put air bags on.
Good luck in your search.