The welding is complete and the steel sheeting on the underbelly of the trailer is now one continuous piece. All 2x3 floor framing is assembled and fastened, insulation installed and subfloor laid. I scored some discounted batt insulation at Home Depot (the lack of plastic packaging apparently makes it really hard to sell) and was able to do the whole floor for $5. I chose 3/4" DryPly marine grade plywood for the subfloor and all floor penetrations are flashed; exposed corners are sealed with Proflex flexible sealant. This should substantially keep water from migrating to any untreated wood around these openings. I ended up with a bit of water leakage due to negligence about poor roof framing, so… temporary bracing. With the DryPly the water pooled up and didn't migrate very far - success.
The debate on sealing seams/edges in RV's:
Proflex RV Flexible Sealant is a permanent solution. It will adhere permanently to anything aside from silicone, it can be painted and will flex with the movement of the camper. Thus far I had been debating the use of sealant versus the use of putty tape and discovered that the difference is the permanence of this flexible sealant; it means anything that might need future fixing/removal should not be sealed with a product such as Proflex. Putty tape offers easier manipulation in those cases. Thus I will be sealing seams such as flashing, roof seams, etc. with Proflex while windows, vents, j-rail and all other trim pieces will be sealed with malleable putty tape.