Weighing your truck fully loaded with passengers and fuel (and without the tailgate) is excellent advice. Then you will know exactly where you stand in regards to remaining cargo capacity.
I don't mean to start a weight debate, nor do I want someone new to truck campers to get frustrated in a hurry.
If your dually is a 4x4, diesel, crew cab, etc. like so many of them are it is probably already quite heavy and doesn't have a lot of payload left. You will find that only campers much smaller than you thought can be loaded while still keeping the truck under it's GVWR. You may wonder what other people are doing to haul around such large, heavy campers.
The answer is they are hauling weights over their truck's GVWR. I would speculate that around 80% or more of truck's hauling TCs are over their GVWR. Is it a good idea? That debate rages thru pages and pages of posts. Just be aware that for better or worse that's how many TC rigs are.
As an example: my rig is an '05 Chevy dually, rated for 11,400 GVWR. Fully loaded with my camper, family, and pulling the boat it weighs in around 13,500.