I had a recent problem with condensation under the mattress for the first time. I found this old discussion, so I'm not the only one. Luckily, I had, for no particular reason, checked under there after the last trip to our damp coast.https://golittleguy.com/forum/#/discussion/2838/condensation-under-the-mattress
One person suggested using a waterproof mattress pad. I used to use one and will try it again. The mattress was wet underneath on my side as well as the dog's side. I had foam pad under the mattress for the first time, so that came out--I had run a dinky heater most of the night with top vent open.
Another had a link to some stuff I'll try if the mattress pad doesn't work unless someone thinks it doesn't work. I wonder, if no air can get under there will make it ineffective.https://liveworkdream.com/2013/05/30/rv-mildew-mattress-problem-solved-with-hypervent-marine/
If all that fails, there is a lattice like stuff, but I hope it doesn't get that far. I will update after experimenting. I'm only 30 miles from the coast, so the dampness here should be close to the same conditions, although it has warmed up a little.