IMO, the best way to figure your load carrying capacity is to weigh the unit dry and compare it to the published GVWR. So, . . . . how do you do that if you don't currently own it? Find someone (hopefully with some experience) in a campground (or elsewhere social media, online, etc.) with a similar vehicle to the one you are considering buying (Class, manufacturer, length, slides, engine, etc.) and ask them what their unit weighs empty/dry. If they have been driving it for a while, chances are they will know. It doesn't have to be exact, just being within 300-500 lbs. is good enough for you to decide if the prospective vehicle is right for you. Hopefully that is acceptable to you. Then, if/when you buy your new unit, one of your first stops should be a scale. Now you will KNOW!
IMO, unlike the aviation industry (I'm a pilot) the RV manufacturers do this intentionally so that you WON"T know what your usable load is. Many people buy RV's not realizing their load limitations and find out later, . . . . after the sale. Big surprise/mistake!
For example: My current 30' Class A gasser on a Ford F53 chassis has a GVWR of 18,000 lbs. So. as a new buyer, . . , . as I was driving it home, (for the first time) empty (OK well with a full tank of gas, a full tank of propane, and me, . . . . . at 205 lbs) I stopped by my favorite recycle yard (they know me well) and I asked them to weigh it. Answer: 15,560 lbs. So now I know my usable load. 2,440 lbs. (that's a good number) IMO, in most Class C's, it won't be that much. DON'T DRIVE OVERLOADED!
Chum lee