towing weight
I'm no longer a 5th wheel person (or perhaps it will turn out that I'm not a 5th wheel person at the moment
, but I'll attempt to provide some insight from when I was.
As Kirk says, the posts here seem to indicate that short wheelbase tow vehicles can have problems towing long 5vers. And short bed pickups definitely tend to have more problems with the trailer 'kissing' the truck in tight turns. An extended pin box and/or the sliding hitch can help reduce the latter problem. I am a big fan of long box trucks, so never experianced any of these problems myself.
Next, as towing up hills, diesel engines seem to be better than gas. So I would say that if the truck works fine on the flats, it will likely be adequate up the hills (even diesels may slow down some). It would probably be wise to have some kind of 'exhaust brake' to help going DOWN the hills. My gas engine was fine on the flats, but was not adequate going up hills, and that was with only 6000 pounds behind my 2500HD.
So the final area to look at is weight. First of all, do not put much stock in the 'dry' weight listed for the trailer. This may be the planned dry weight, or even the actual dry weight from the factory. However, it won't include anything added or changed by the dealer or previous owner. When shopping for trailer weight, use only the GVWR of the trailer, because that is the maximum weight it is designed for.
The only use for the 'dry' weight is to give an indication of how much stuff you can carry in the trailer, or CCC. This is GVWR - dry weight. If this is anywhere near not enough for you, make sure you actually weight the trailer empty before committing to buy it. And weigh it loaded for travel before setting off on a trip.
So how much trailer to get? I'm a bit leary of that 15,400 weight. In the past, at least, the claimed trailer weight is with a truck with no accessories installed, no cargo, no passengers, just a 150 pound driver and a bit of gas. Certainly you can use their numbers as a guide, because they probably won't outright lie to you, but it is wise to look for trailers with a good safety margin for the real world. For instance, you mention a 2500 pound pin weight, I think. You would probably need to subtract the weight of the hitch, and anything else in the bed of the truck, from that weight.
Really, the best way to figure out what you can tow is to take your truck, loaded with all gear and people just as it will be when you tow, to a scale and weigh it (front axel, rear axel and total weight). Now you can subtract the total weight from its GCWR (Gross Combined Weight Rating) to find out how much trailer you can REALLY tow. And subtract the rear GAWR (Gross Axel Weight Rating) from the rear axel weight to find out how much pin weight you can really have.
Note that pin weight 'tends' to be 10% to 20% of the trailer weight, and assume 200 pounds for the hitch unless you have the real weight. And the smart driver will try to stay as much as 20% under his ratings so as to never give even the appearance of being over his rating limits. Being over limit may or may not lead to accident, premature wear or breakdown, but does tend to attract attention from highway police in some states, and lawyers after any accident...