I'm not sure which fifth wheel you purchased, but it sounds like too much trailer for the tow vehicle. If it was listed at dry
weight of 4800 lbs, I think it would be a safe bet that it was way more than that when traveling. That would make it too much trailer for your truck, I would think. I'm thinking that it's probably not a molded
fiberglass 5er like the
Scamp or
Quantum, is it? They are much, much lighter than that and you probably wouldn't have any trouble towing with your truck.
Also, I'm not sure if it was new, used, or purchased from a dealer. If it were me (I stress that this is what I would do and not recommending that you do it) and it was purchased from a dealer, I would notify them immediately and explain the situation to them. That you were told that your truck would tow this trailer and now find that it won't.
The longer you wait, the more you lessen your chances.
Salemen who are unknowledgeable or, worse yet, care more about selling than safety really get my dander up.
As I said, this is just my opinion FWIW.
Good Luck