Given no leaks in an RV's roof ... it's not rain, snow, or sleet that ages or damages it.
It's the sun's UV rays that ruins painted surfaces, dries out sealants, and rots materials that age and damage it. Keeping your RV out of the sun as much as possible during either long camping stays or long storage periods - is a number one priority in extending the roof's and sidewalls' life. Also, sitting hour after hour in the sun during the warm months when not being used - in which the interior temperatures rise to high temperatures - also dries out interior adhesives and slowly heat-rots interior materials.
The sun is not an RV's friend - unfortunately so for those owners who must camp with their RV out in the sun for solar power - although roof solar panels do shield the roof areas right under them from UV ray damage.