Here is a couple of other ideas:
Hypervent used in marine applicationsThis is what I used, it is to much in a roll for one person so I went in with a couple of other people and I bought a roll and we shared it. It is meant for cedar roof applications but it does give air flow under the mattress and this is what you need. Since I got mine we have not been out yet so I don't know how it is working.
Maybe insp1505 well post on here about how it is working for him. He is full timing in his NL in OR where they have a lot of humidty and was having the moisture problems under his mattress. He got 2 pieces and doubled up on the amount of cedar breather under his mattress.
Cedar Breather