I've used reflectix for years under the bunks in a pop-up to good effect. There is also a solution popular in the boating world (If there is a place for condensation and humidity it's on a boat)
Simply buy a sheet or two of plastic lattice work (like you'd find in a garden fence) and place that under your mattress. Works like a charm!
https://www.lowes.com/pd/Deckorators...ttice/50280313