<That said, use a marine grade finish if you must use plywood,
& be prepared to refinish every couple of years,>

Years ago, many years ago, I had a Wellcraft power boat, and I took the Coast Guard boating safety class. The instructor made the comment that he never could figure out if wooden boat owners really liked boating or just like to paint. I also knew a guy with 26 foot wooden cabin cruiser, and he pretty much confirmed that, it was a maintenance nightmare. But I guess the same could be said of  Vanagon owners, do we really like campers/vans, or do we just like working on cars?     Jeff