We have $20k, which used fiver under 29' that can dry camp?

I don't see a problem weight wise if you're looking at 28'. Even with a 4 season 5th wheel your pin weight should be under what is allowed by your RAWR (considering you have the proper tires). You may need to consider some airbags if it squats more than you like.

I would get the truck weighed first, so you know what you're dealing with, and how much capacity you have for pin weight over the rear axle.

Wish you luck in your search.