Lots of responses of what to do to try to keep the air dry but nothing on what to wipe surfaces down with.
Here you go. I live in Bermuda where quite frequently the humidity is over 90%. No RVs here but I have been storing a 32' boat for quite some time, years, in a location that does not get much sun & can be extremely damp.
Before mothballing the boat, knowing it was not going to be used for quite some time, I wiped down all hard & smooth surfaces with a clorox/water combo 50% & left it to dry. The residual chlorine left on the surfaces kept them mildew free for well over a year.
Do NOT put on fabric.
The clorox/water combo works for other surfaces as well. Big winter project for me this years was cleaning all of the painted & exposed to weather rafter feet around my house. Dawn dishwashing liquid works well for that part but while at it the eves of the roof were somewhat moldy as well. We don't use roofing material like you do in N America. It is a much more substantial stone slate material to stand up to hurricanes which must be cleaned & recoated every few years. Again a clorox/water 50% mix works well to kill the mid cycle mildew. Spray or brush on. Leave it.