Hello, I have a 1985 F250 and I was wondering if a 3,000 pound camper is still under the max payload for the bed?
Look on your door jamb for the weight rating sticker. It will tell you what your Gross Vehicle Weight Rating (GVWR) is, along with your Gross Axle Weight Rating (GAWR) for both the front and the rear axle. These numbers represent the maximum weight that your truck's allowed to be at.
Then, go weigh your truck. Make sure you have a full load of fuel, whatever tools you normally carry, etc, but nothing in the bed that you don't carry as a matter of routine (i.e. no dirt bikes, but yes to your toolbox if you regularly keep it in the truck bed).
Subtract the results from your weighing from your GVWR and you'll have the total payload capacity of your truck. I'm pretty sure that, for a regular-cab F-250, the total payload capacity's going to be considerably lower than 3,000 lbs.
Things get more complex (axle weight ratings, tire weight ratings, etc), but that's the short version.
For what it's worth, I'm wary about putting a full-size slide-in camper on a SRW truck...even if the camper's within the weight limits, with something that tall on the back of your truck, I like the idea of the extra stability that training wheels allow...