Well we went both ways, started about four years ago with a 89 F250 with a 7.5L gas, not sure of the gears likely 3.55's and a C6 tranny. Bought a 86 King of the Road "fixer" cheap to test the waters 28'. During the tow home it was clear the truck wasn't up to pulling this trailer, struggled with hills. We spent a couple months fixing the 5er, about 9,000# loaded. We for the next year we debated fixing the F250 or getting a newer diesel. Well the Diesel won out we picked up a 01 Ram Quad Cab 5.9 with a 5 speed, what a difference!! It pulled that King just fine.
Then we started getting the bug for a bigger 5er with at least two slides, last year we found an 05 Copper Canyon 293 FWSLS, and we both liked it very well, so we bought it. It has a GVWR of 12,500#, we have it very well stocked and it weighs in about 11,000#. Our 2500 is a Camper special with one ton springs, that and a DS Power Puck were all we needed. I see lots of threads on people using air bags to fix "sag" and ride. I fixed the ride by replacing the factory original shocks (255,000 miles) with a set of Bilstien 5100's. I can't really add air bags for the "ride" as I don't sag enough to be able to add and air to them! Currently in the process on installing a Pacbrake, and I will have all the TV I need.