It is very common to grossly underestimate final camper weights.
First you need to be very careful about the actual specs from the manufacturer. Quite often the specs they provide are for a basic unit which does not include typical accessories such as A/C, awnings, microwave, larger refrigerator, TV, etc. Next be sure to include the wet weight if that is not included by the manufacturer. Even just 30 gallons of water and a couple of propane tanks can add another 300#.
Next you have to add heavy items you might carry such as a generator, fuel, extra battery, and any upgrades such as a foam mattress. Include upgrades you might need for the truck such as a bed mat, brackets, tiedowns, leveling blocks and any suspension upgrades.
Finally you need to add in an estimate for all the items you are not going to count individually. Food and drinks are really heavy. Lots of small items add up. That would include kitchen gear/pots/pans/plates, towels, bedding, clothing, BBQ grill, electronic items, hobby gear, cleaning supplies, extension cords, maps/brochures/travel guides, binoculars, walking stick, etc. It is really easy to have all of this add up to 750-1000# depending partly on the number of people and length of stay. Some people even add a pile of firewood to the cargo.
