Kathleen,
We've got some folks on here with experience re-sealing the roof. Unfortunately, I'm not one of them. Jim just provided some good advice on cleaning the roof. I think the other important thing to do would be to carefully inspect the sealed areas for evidence of cracking or separation. Storage conditions for the RV may make quite a bit of difference in when or how soon resealing is needed, so what was right for one may not be right for another. The main thing is to prevent water intrusion, so if all your seams look smooth and well-sealed you may be good to go. If you find cracks or areas where the seam is loose, then resealing those areas is certainly called for.
I have seen some pretty good YouTube videos on how to reseal, so that is an option for information as well.
Mike