I do not know how GM comes up with these calculations for slide in camper rating.
How do they know how much the slide in weighs.
Some slide in campers weigh as little as 1,500 lbs, others can weigh 5,000 lbs+.
With a 3500 truck you should be fine as long as the camper does not go way over the truck payload rating.
Even if you are under the payload rating, an aftermarket rear sway bar and rear air bags make for a much better ride with the camper on the truck..