Density counts...so does water content. Four feet of snow from one snowfall is a lot lighter than four feet total over a season.
Dunno what "standard" there is if any for Rv's, but typical residential roofs are designed to withstand 30 pounds of load per square foot, which is roughly four feet of fluffy snow, two feet of dense snow, or six inches of water (ice). If you want to know how much weight is on your RV roof right now, just go out there and scoop up a cubic foot of whatever's up there. Weigh that, multiply by the depth, and you'll have the per-square-foot load actually on the roof.