Personally, I'd sell and pay off the 1st trailer before heading to the dealership for the new 5er. Nothing sweeter than a paid off toy.
That said, check with your tax guy but you may also be able to finance and claim your 5er as a 2nd home (15 year mortgage and deduct interest).
Not to be rude, but a "decent truck" may not be enough for a 5th wheel. Since 5er's carry a heavy pin weight 20-25% of loaded trailer weight it takes more truck to pull a 5er than a Hybrid or TT for that matter. The truck will run out of cargo capacity well before it runs out of towing capacity. A pre 2005 truck will also have much lower capacities than a newer truck.
Run the numbers and be sure you are not complicating issues. Don't trust the salesman, do your own math.
Good luck.