That is a catch 22.
Would you really ask people "I want to wash my motorhome, but should I fix the water leaks first?" How many would say "Go ahead and wash it, not to much water will leak in?"
I would say wait a few days to wash it, and be careful not to get the fresh sealant to wet. It is still curing, even on day 3 or 5. No power washing that area for a couple of weeks. .
I coated my entire roof with a EPDM sealant, all cracks, and vents, sidewall to sidewall. Now I do not have white streaks going down the sides! That made the coating priceless! The factory EPDM (at least when mine was new in 1997) was designed to "Shead" a little of the white stuff every year, and keep it looking new. After 14 years, most of the white stuff that was left came off while washing it in prep to coat the roof. After applying the coating, no more white streaks! I should have done this years earlier!
Of course if you want to wash the other areas of the RV, go ahead! Rain probably will not damage the sealant, but it will not have soaps, or other things trying to remove the dirt.
Good luck,
Fred.