Have you allowed to build up on your rig before climbing up and clearing it off? Got real worried with 18+ inches of Sierra cement and finally did it. Wasn't easy but what a relief!
Snow load design for RVs is a good question. Houses are built different ways depending on where they are due to anticipated snowfall but since an RV has wheels, no one knows where it will be. Realistically, the roof span is pretty small so it's not hard to just over-build them. Of course, over-build is not a term typically associated with RVs so who knows? Good question though.