Your Build looks really cool. I work for a company called trailerworks, we build Vintage inspired teardrop trailers, <shameless plug www. retrotraveler.com> we make then pretty similar to your design, I just thought I would share a few of our “trade secrets” we have learned along the way. First off BASF makes a product called Sonolastic NP1, it’s a tube sealant available in a wide range of colors and about 4-5 bucks a tube, and you can get it online. It is will stick to just about anything its take about 12 hours to fully dry but it remains flexible for ever. Just use denatured alcohol to clean up.
Also we use a Butyl backed foil tape anywhere where the wood sidewalls/floor and frame meet.
And if it is not too late, while your painted plywood bottom will hold up for some time, if you want it to last forever get a piece of Galvalume sheet metal and put it underneath your wood base, the Galvalume is light and not the expensive and will last forever.
I hope some of that helps make your build go a little smoother. That is going to be one sweet trailer when it’s done!