Your roof should be an EPDM membrane material glued down over a substrate of 1/2 to 5/8 plywood or composite (depends on the year of manufacture and model; you don't mention what CruiserRV product you have
If you mean by "put weight on it", can you walk on it? If that's what you're curious about, then, by all means you can walk on it if you are under 400 lbs or so... (if you weigh that much you shouldn't be on a ladder to get up there!
) If you want to know if you can put some kind of luggage rack up there and tow it with a load on the roof; NO. The roof will take walking on for maintenance and cleaning, but, it was not designed to take additional long term loading, nor is the EPDM sturdy enough to have a load sliding around on it while traveling.
If you want to get up there for an "inspection" (and you should do that at least annually, I do mine twice a year at least) or to install a cover over a vent, etc., or put a cover on the trailer; be sure to wear shoes with soft soles, don't "twist" on the balls of your feet and don't step close to the edges of installed skylights, vents, pipes, etc. as you may cause enough "flex" to break the seal and allow moisture under the sealant. Also don't get too close the edge...it is a long way down.