I suppose it would depend on the type of mattress and the climate you are in. I have a foam mattress that sits on a plywood base. I keep a memory foam topper on my mattress.
I spend winters in southern AZ and summers overlooking Lake Washington, near Seattle. The only excess moisture problem I've ever had was caused by Sena setting an open bottle of water on the bed and letting it tip over.
I would think that if condensation was a problem in your RV, the mattress is just part of what you'd need to protect. A dehumidifier sounds like a better solution to me.
Barb (RVM18) with Morkies Lily & Bebe RIP Sena FMCA#F466348
"Homer" ‘11 Shasta Cynara, pulling "Ranger" '97 Ford Ranger toad
The Journey is Our Destination. Full-timer May 2011 - July 2021