Get the house you want and then buy the vehicle to pull it. Pick out your 5th wheel first then go truck shopping. IF you buy the truck first then you are limited to what that particular truck can CARRY.
Biggest mistake 5er owners make, in my opinion, is buying the truck and then trying to match a 5er to it.
BTW I have a 2500 HD Crewcab SRW Duramax it is not the 2015 model so maybe the new ones have been beefed up. My payload is 2500lbs not 3000lbs.
I made the mistake of buying my 5er after buying my Ford 250 Powerstroke. It just could not carry the load. The Duramax can carry and pull the load. It acts like there is not even a trailer back there---except for the mileage :D