We have never had a problem. Each day when camping we spray the walls the Tilex Shower Spray. Before storing I clean the walls with Aquachem cleaner and dry them.
We store with the end windows open a little plus the two ceiling vents (which have the Maxxair covers) AND we cover the RV with a Sunshield tarp to minimize the sun effect.
You might also try spraying the walls with Concrobium before storage.