A '99 wouldn't have any sort of "payload sticker." They only recently started doing that.
The REAL way to find out how much payload you have is to take the truck to a scale, and have it weighed. Subtract the truck's scaled weight from 8800lbs, and that is your payload.
It probably isn't 3800lbs, but should be more than 1500lbs. I would expect somewhere between 1800-2500lbs.
3800lbs is the "added helper springs, upgraded to 19.5 commercial truck tires, don't care about factory ratings" payload capacity.
Older campers tend to be lighter if they're original. Once someone goes in and does a "remodel" probably using conventional materials and no consideration for conserving weight, that's all out the window. Any original data plate is meaningless, and odds are the camper is much heavier than originally built.
One consideration you should have with an extremely old camper like that is FITMENT. That 1978 camper is designed to fit in a 1978 truck. Ford built trucks with basically the same box dimensions from the 1970's up through 1996. In 1997 Ford shrunk the box size on the F150 and followed suit on the F250/F350 in 1999. So your box is smaller and the 1978 camper may not fit without heavy modification. Be aware of that.
Putting 10-ply tires on half ton trucks since aught-four.