I often wondered about the underside of the a wooden floor, Rob-Jax (deceased) the Famous Eriba Oracle wrote this on practical caravan website in 2005.
Thoughts on a modern caravan floor: Our Eriba floor is a standard plywood / foam / plywood bonded sandwich, with a bottom breathable membrane and protective coat. If you have this type of floor, don't cover it with any sealant - it needs to breath.
Attempts in the 1970's to seal Eriba floors with an all-round waterproof coating were not successful - many 60's Eribas have their original plywood floor and all 1980's onwards will be original - but probably not those from the 70's. It's nice to have a group of long lived caravans on hand - then you can look back at what was a "good idea" 30+ years ago and see if it really was.