Here's a simple accurate way to get it right and to account for all options whether dealer installed or factory. Load up the truck with everyone that will be traveling in the truck when you tow that fifth wheel. Head over to truck stop that has a CAT scale (ie Flying J, Pilot, Loves). Fill up the gas tank and pull around to the CAT scale to get weighed ($12). The weight ticket will show the combined vehicle weight (my 2500 Diesel is 7520#).
Your Payload is equal to 10,000# MINUS your ticket weight (My Payload is 2,480#).
From your Payload, you will subtract the weight of your fifth wheel hitch. If you don't know what its going to weigh, pick one on eTrailer.com and check the shipping weight.
That net payload will be your guide as you look at fifth wheels and check their dry pin weights. I say guide because as you already know, the stuff you carry in your trailer will change that dry pinweight some.
You said you were looking at used trailers, ask the seller if he has a recent weight ticket (you might be surprised).
Lastly, for a ball park compare, I drive a 2015 GMC 2500HD Diesel. After hitch and some stuff I added (nerf steps, airbags) my ticket available payload is 2010#.
My 2016 Wildcat 295rsx is 30 feet long and the dry pin weight is advertised at 1560#. Last time I checked at the CAT for both Truck and Trailer, my ticketed pinweight was 1960#, just barely below that 2010# I measured without the trailer in tow.
When we were shopping to move to a 5th wheel we did pretty much just what you are doing. We found that Ultra-light Fifth wheels were mostly where we met spec. Rear Living units seemed to have the lightest advertised pin weights.
I posted sticker pictures and stuff within my Registery entry at the Forest River forum. Some of this may be relevant for you. Here's the link:
hbillsmith Wildcat RegistryGood luck and hope you find what you want.
