While the truck max payload is important, and everything in truck/trailer will add to your GCVWR (which is a drive train limitation), your most important number of concern, is your RAWR/tire max load. With a FW, nearly all the pin wt, plus hitch wt, will be over your rear axle.
Agree with Lantley, you are adding 4 6 volts (300#s, plus 60#s propane) likely right behind the pin, so this ups the percentage some. Most cases, a FW pin falls between 20-25 percent when loaded.
If you weighed your rear axle, truck full of fuel, everything in the truck you'll be taking, add 175 for the hitch (don't forget the wt of rails), you'll know how much more pin wt the rear axle can carry.
My guess...adding 2,500 pin wt, will put you close to RAWR, after hitch, and everything else in the truck. With your FW loaded to GVWR 11,700, your pin could be upper 2K, at 25 percent pin. Most likely you should be able to keep pin wt at 2,200 or less.
Jerry