This is your killer. Pin weight plus hitch plus people and stuff needs to stay under 1618 which is REALLY hard to do with a 5th wheel. It's honestly pretty hard to do with a TT over 28 feet unless it is super duper light (maybe we can print some 3d plastic cabinets? lol).
I have a Coachmen Freedom Express 292BHDS (dry weight 6104) and I had to do a little weight balancing to get under my payload limit. Bikes and Ice chests need to go inside the trailer, near axles to not impact my payload as much. Also we are two adults and a 30 pound dog. We never travel with more than 2-3 gallons of water in the fresh tank (just enough to flush the toilet and wash hands if needed). If I had a truck full of kids I'd be over the payload. If I had a camper full of "stuff" for that truck load of kids, I'd definitely be over!
If you persue this further be ready to do the math.
Don't listen to the sales guy.
Ignore published dry weight numbers. Look at the yellow sticker on the actual trailer you're looking at, and the max weight of the trailer.
Calculate a REASONABLE amount of weight you will add to the 5er's shipped weight.
Calculate appropriate pin weight for your LOADED 5er weight. I hear 5er owners saying pin weights are 20-25% of the trailer weight sometimes.
For me, I think the restrictions to getting a 5er I could safely tow would be too much. I'd rather get everything I want in the quality level I want in a TT.
The next truck will probably be a 1 ton. Then I will re-consider a 5er!