everyone, i need help with a truck/truck camper questions.

well there are plenty out there made fore all 1/2 ton trucks.

like campers i've seen designed for dakota or tacoma for example. some were legal,and others were on the charts saying "made for tacoma" but its dry weight was 1300lbs...even tho the payload apparently on a tacoma is 1150lbs.
so i could easily have gotten it, but it'll be over my payload.

so I'm just trying to get a grasp of whats aloud and whats not.