Originally Posted by Don_Monaco
... but then he said I could install air bags and have 3500-5000lb cargo weight.
Your salesman is either ignorant or stupid, or else he's willing to lie just to make a sale.
You will definitely require air bags or some sort of stiffened rear suspension if you attempt to haul that much hitch weight with an SRW pickup. But those suspension stiffeners do absolutely zero to increase the payload capacity of your pickup. They do not increase GVWR or rear GAWR of the truck.
What air bags do is bring the headlights back down from the sky, so you can tow at night without blinding oncoming traffic. And they raise the rear end of the heavily-loaded truck so it looks better - more level. Plus you want the rig level with the road so the trailer brakes will function properly, and the rig will tow straight with less tendency to go into an uncontrollable sway.
The only way I know of to legally increase payload capacity is to jack up the old truck and run a heavier-duty truck under it.
There are modifications you can do to increase GCWR (pulling power), but not payload capacity (weight hauling capacity) which is based on GVWR. GCWR depends on the power and torque of your drivetrain, and there are mods you can do to increase the power and torque of your drivetrain. Replace the ring gear and pinion with a numerically higher ratio. Hot-rod the engine with a towing tune. Improve the intake and exhaust systems to make them free flowing. Install smaller-diameter tires to in effect change the axle ratio to a higher numeric ratio.
But GVWR and rGAWR depends on the weakest link in your truck, including frame, wheels, tires, shocks, springs and other suspension parts. And brakes. The truck brakes have to be engineered to stop the weight on the two truck axles. The trailer brakes have to be able to stop the trailer, but if you exceed the GVWR or rear GARW of the truck, you may not have enough braking power to stop the rig in emergency conditions. And it's those emergency conditions that you are planning for when matching trailer to tow vehicle.
Does it make much of a difference between short bed or long bed on 5th wheel. I can get a short bed crew cab in the garage.
Yes, it makes a big difference in the hitch you need. With an 8' bed you can use an ordinary 5er hitch. But with a shorty, you need a sliding hitch. Sliding 5er hitches are heavier and cost more, but if you use them correctly they will prevent the front of the trailer from crashing into the back of the cab.
There are basically two types of sliding 5er hitches - manual and automatic. Automatic costs a lot more. But with a manual slider you must be wide awake every time you put the tranny in reverse, and NEVER
back up without first sliding the hitch. You probably cannot turn the rig sharp enough to have trailer to cab contact when going forward, but in reverse you can jackknife in a heartbeat, then CRUNCH!
So the answer is the PullRite SuperGlide automatic 5er hitch. Fully automatic slider, so you need not worry about forgetting to slide the hitch. They come in three different mounting types: Original under-bed SuperGlide bed rails, OEM type that will plug into the 5er prep pkg offered on new Ford and Ram trucks, and so-called "industry standard" type that will use the Reese 5er bed rails that are above the bed floor. So if you buy a used truck that already includes the 5er bed rails, you can buy the correct SuperGlide that will plug into the bed rails.
If you're a Dodge fan, then Ram still makes shorty dually pickups. Ford doesn't, and I don't think GM does either. Ford offered them way back when, but they were very poor sellers, so Ford stopped offering them.