I have no direct experience for either Chuckwagon or Steve and Jan, but I'll share my uninformed thoughts.
Chuckwagon - Hardware stores carry various dessicant products that should help to prevent mildew while stored, by taking moisture out of the air. I seem to recall that there is a brand call Dri-Z-Air or something similar, and there are lots of options on Amazon.com. They are stocked with the household cleaners and probably other locations in the big box stores. If you can store it indoors, I would recommend keeping it opened up (doors open, screens up, if you have them). If you store it outdoors hopefully you don't have to store it in the open under a cover, as I suspect that could trap a lot of moisture.
Steve and Jan - Interior climate controlled storage is probably overkill, except in areas of the country where it is humid a lot (being near a coast might put you in that category). If your boat will be moored in salt water, indoor storage would seem to make more sense, since salt air has a corrosive effect on the aluminum skin of the camper. We stored our camper in an unheated garage in Boise, where the outside temps could range from 5 degrees to 110 degrees. We had no issues, but that is a very arid environment. One downside of outdoor storage is that the camper could be visible from the street, giving someone plenty of time to figure out how to break into the storage area and steal the camper.
Hopefully there are members with direct experience with your issues who can give you more food for thought.