Test your slide out roofs and see if you can do it. Put some weight in the middle of the slide out and see if the roof is soft or firm. Add more weight and if it's still firm you are probably OK. As a last test, get up there and put your entire weight on the roof and if it's still solid, then you know.
As far as the slide itself not being able to support your weight on the roof? Well, that's no different than supporting your weight standing on the floor. More than likely, the roof is made of the same material as the floor (plywood). The walls are probably aluminum with Styrofoam.