If your objective is to stay within the manufacturer's ratings, you really can't go with brochure information on either the truck or the RV. To explain, the truck's "trailer tow rating" is obtained by subtracting the curb weight of a stripped base truck with only a 150 lb driver from the truck's GCWR (gross combined weight rating). Correspondingly, the "dry weight" of the 5th wheel doesn't include propane, your cargo (food, clothes, "stuff", etc.) and, many times, any options. Many RVers look at these brochure ratings, put a rig together and then find that they're exceeding their truck's GVWR, GCWR, rear GAWR or some combination thereof.
The problem with any SRW truck when towing a 5th wheel is the high percentage (20% to 25% can be typical) of the 5th wheel's weight that is carried by the truck as hitch or pin weight - this weight counts against the truck's GVWR, and the SRW truck has a lower GVWR than the dually, despite the fact that a given SRW truck, being lighter, may have a higher manufacturer's "trailer tow rating".
If you have a realistic idea of the true, actual laden curb weight (LCW) of the truck(s) you're considering or will be using, you can use the following equations to size an RV (5th wheel or TT) that the truck can handle. LCW is the actual curb weight of the truck with driver, passengers, cargo, full fuel tank, options, accessories, trailer hitch, etc. This weight can be obtained by loading up the truck and weighing it on scales that are available at most truck stops - it should cost $10 or less to weigh the truck.
Truck's GCWR - Truck's LCW = maximum allowable
total weight of
loaded trailer
Truck's GVWR - Truck's LCW = maximum allowable
hitch or pin weight of
loaded trailer
If you don't know the loaded weight of the 5th wheel you're considering, a conservative approach is to use the 5th wheel's GVWR as the total weight and 20% of the 5th wheel's GVWR as the hitch or pin weight of the loaded trailer in the formulas above.
Good luck with your RV search and welcome to iRV2.com.!!
Rusty