I believe the sticker in the glove box uses the camper weight rating assuming all seats are filled with passengers of 150# each. Thus, the total payload rating would potentially be some hundreds of pounds higher. Conversely, if only 1-2 passengers are in seats, the camper weight rating would go up150# for each empty seat.
For example, using my 2001 9200 GVWR GMC truck listed in my signature, the glove box sticker states a camper weight rating of 2416#. It has six seat belts so that rating is figured assuming 900# of passengers (and a full tank of gas). Theoretically, my truck could take me (at 180#), a full tank of gas, and a camper that weighed 3136#. From that, subtract the weight of each additional passenger. Add my wife and now I can carry a 3000# camper.
I've only had one truck camper on that truck, a 1989 Lance Squire 4000 that had a dry weight of about 1800#. The only time I ever weighed the whole package was shortly after 9/11 when I had my wife, two young kids, full water and propane, and more gear than normal as we were heading to LA and didn't have any idea what was happening in the country. Point being, we were loaded for SHTF travels. Both axles were under their max and the truck/camper/passenger/gear total weight came to about 9100#, 100# under the GVWR.
The only sure way to know what's what is to take the truck with full gas and passengers likely to be coming along and get yourself weighed. Subtract that from GVWR and you should have a pretty good idea of what you can carry. But you also have to weigh the axles and make sure you have load rated tires appropriate for the load you're carrying.
