Ya this is an old topic, but we were all new to truck camping at one time. The payload works by subtracting the mass or curb weight of your truck from the GVWR. The GVWR can be found on the inside of the drivers door. This will tell you how big of a truck camper you can legally put in your truck (keyword legally). Keep in mind that this does not include cargo, kids, or gas which add up REALLY fast.
That payload number does seem low; it would be how much you carry in your truck (cab, bed, fuel tank). The axle ratings can also be found inside you drivers side door.
Suspension modifications (ie: air bags, sway bar, etc) will make the truck camper more stable but will not increase the GVWR.
If you take a look around the site there are a lot of people with 3/4 tons here. Maybe some of them have exact numbers on their weight's and can help you out.
Best of luck, I'm sure you'll find a camper or make it work!

Edit: Grammer mistakes.