There is a product which got a good write up in a UK yachting paper about a year ago. It is called Ventair. Their main market in the U.S. seems to be R/V and campers but in the U.K. the concentrate on the marine market for some reason.
Anyway, their product, to use their own words, is "a three dimensional nylon mesh core sandwiched between two polyester covers. Having a unique construction, Ventair is unaffected by moisture, paints and varnishes. Ventair creates an air space, allowing air flow under mattresses and behind cushions, thus reducing the damaging effects of damp, mould and mildew. Ventair is flexible and extremely resistant to crushing. Movement on the mattress will flex the core so it acts as a pump to exchange the air under or behind your furnishings. The natural resilience of the material adds to the comfort of the bed."
Find out more at www.ventairusa.com
As I have not used the product, I cannot endorse it but as I sail in cooler northern climates where condensation is a very real problem I indent to test it this coming season.
It may well be the solution to your problems too. Good luck with it.