This is a review of a product called “Hypervent”. It has been mentioned in previous posts, but since we recently installed this under our RV mattress I wanted to pass along my thoughts and recommendation.
We have been living full time in our trailer this winter in the Seattle area. It’s been a particularly damp winter with a significant cold spell. As a result, we were plagued with a damp underside of our mattress and bedding. This, even though using a dehumidifier and using limited amounts of propane for heating and cooking. I pulled the mattress away from the front wall frequently too, all to no avail!
Yes, I laid “Reflectex” under the mattress and even the side and front wall surrounding the head of the bed. This may have added a bit of warmth, but did NOTHING to stop the condensation. If anything, it just allowed the moisture to pool (not like swimming pool, but like seriously damp 
).
Enter “Hypervent”. Simply, this is a 40” wide x 3/4” thick pad of spun plastic material covered with a breathable synthetic cloth. It is sold by the foot-length. It has been used a lot in boats. I purchased it from https://www.hyperventonline.com/ , which happens to be a Seattle (Ballard) upholstery shop. Check out the website for a complete explanation, ordering amount information, and comparison to a cheaper (inferior?) look-alike called “Hypervent Aero-Flow.”
Hypervent WORKS! No condensation under the mattress for the two weeks since installation…none…at all!
Installation: I left the Reflectex in place. Ordered 10’ of Hypervent and cut it into 2 x 5’ pieces. This covers the Lance queen bed platform and allowed me to run it up about 4” on the front wall below the window. I think this “elbow” of Hypervent curving down and under the mattress gives it better air flow/ventilation at the very head of the mattress. I used a few strips of Gorilla duct tape to “seam” the two 5 ft pieces together and a few strips to secure it to the plywood bed platform. 15 minutes tops? To satisfy @Capt PJ here are a couple photos. The first one is under the front window and the second one is at the foot of the bed. It is seriously strong stuff. Even my 220 lbs kneeling on it didn’t crush it. It has zero effect on the bed cushion or feel. Just good stuff! Am I Hyperventing? 
