Above 30' a 5er will likely be so heavy when loaded that it will overload a 3/4 ton truck.
Know three things about your truck. GVWR gross vehicle weight rating usually on a plate on the drivers door post. GCWR gross combined weight rating, mine is in my owner's manual. Then you need to know the agtual weight of the truck as it will be when you are on the road towing. Load it up with Mom and the kids, dog, etc, full tank and then weigh it. Add the estimated weight of the hitch and you have the truck weight. Now you can come up with some numbers to use when shopping for a 5th wheel.
Truck GVWR- truck weight equals the maximun 5er pin weight. %er pin weight is often 20% of the 5er's GVWR so you can compute the trailer GVWR you do not want to exceed.
Truck GCWR - truck weight equals the maximum 5er GVWR you can safely pull and STOP. (Note the 5er GVWR determined from the pin weight calculation will usually be below that of the GCWR calculation.
I would use the 5er GVWR in doing these calculations and not listen to a sales person who will assure you that the 5er will weigh well below it's GVWR. I doubt it.
If I had done this properly I might have bought a 1 ton.
2000 Quad Cab 2500 Auto w/4:10 rear end. DTT 89% VB, TC, Mag HI-TECK Pan, Smart Box, EZ, PAC Brake, Pillar gage mount with EGT, Boost, and Tranny output temp, Miller Manufacturing Truck Bed Cover, with RBW 5er hitch, and
Rigged to tow a 29' Automate 5th wheel.