I recently bought another Alaskan Camper to refurb and the owner had a small heater he had built for the winter...the camper was purchased in Gig Harbor Washington...in the woods...very humid/cold/drippy.....and the interior of the camper was as dry as a bone...stored outside under an aluminum cover open to the weather.....with this little heater inside
Heater: porcelain base light fixture, mounted on 8" plywood square with a small column of aluminum sheeting coiled to about 6" and mounted to the plywood with a small airspace at the bottom....mounts are small angle brackets and the aluminum sheeting is screwed together along it's side...simple, effective and it works....of course the interior is real wood plywood....as are the cabinets (some solid stock too)
Wood and water do not mix....any wooden boat owner will assure you of that...it's a constant maintenance issue....but the very best way I've found to preserve interior woodwork...is entirely old school....sand smooth, white shellac seal, when dry, sand smooth again....and 3 coats of spar varnish sanding between coats ... at least an overnight dry for each coat....wet or dry 300 grit sand the final coat and 0000 steel wool for a satin finish that is not only as close to waterproof as it gets but you can use it for setting wet things on without stains appearing. cloths, glasses, bottles et al
I've used this on furniture for a very long time (50 years plus)...and I've never had a stain....exposed to the sunlight it will uv degrade and must be kept up like any exterior surface finish....
any organic material will develop fungus/mold and rot before your very eyes...the smaller the pieces of organic material the faster it goes...hence, flakeboard's instant degradation...the vinyl film just helps to retain the moisture in the base
mold spores are one of the most prevalent things on earth...they're everwhere