25% of 14000 is 3500.
So on a theoretical max weight of 14000 you're looking at a theoretical max pin weight of 3500lbs. That doesn't even count the weight of the 5th wheel hitch.
Your average everyday 2500 series truck is not rated to handle that even ignoring GVWR and just going on rear axle and tire ratings. They still tend to have 3000-3200lbs of payload before you hit the limits of the rear tires.
The truck will need to have suspension and tires for a 14000lb trailer.
Simple fact of the matter is, you're not going to find a 3500 SRW on dealer lots right now. You can barely find 1500s and most dealer lots tend to have 25 1500's for every 2500, and 25 2500s for every 3500 SRW.