My opinion (could be all wet) is that manufacturers list the weight of their units with nothing inside. Slides, yes, because it's part of the structure but nothing else and everything weighs something.
I think if they listed the actual weight (optioned out), they'd loose a lot of sales because the camper weight exceeds the payload weight of most trucks.
Passed a guy today on I94 with a big Lance on a long bed Chevy single rear wheel and it was doing the rear squats. I wondered if he had any idea what it weighed or if he had checked his tires lately...
Pretty sad when you have to buy it and then go weigh it and then find out you don't have enough truck to haul it safely.