I don't even think there is an epoxy that will truly bond to HDPE or UHMW-PE.
If you rough it up enough you can achieve a "mechanical bond" but it won't be reliable.
I have fiberglassed to PVC products but its still never a reliable bond.
From the marine products side there are some high density foam sheet goods and other "core" products that are strong, lightweight and water proof.
True Marine plywood is over $100.00 a sheet if you can even find it. Pressure treated plywood I have never found a sheet that wasn't warped to high heaven. And polyester resin doesn't much like sticking to that either from my limited experience with it.
Whatever you do stay away from Balsa!
Yes it's in boats, but its there because it was cheap and light
and there were few alternatives. Not because it's a reliable sound solution
Asphalt emulsion on a good quality plywood, in a travel trailer should have no problem lasting 50+ years. As Long as its not subjected to long term water.