For the bottom panel of your TD, the one that faces the ground and sits on top of frame, I have several questions:
1. Has anyone used exterior grade paneling for that exposed piece?
Yes, I have seen that.
2. In coating the underside, have you also coated the end grains with the same coating, or used something different for aesthetic purposes?
Best recommended practice is to treat all exposed ends with polyurethane cut 50% so it soaks in good. See rot doctor...
http://www.rotdoctor.comSecond best and the classic method for underside and ends is to use a roofing tar- emulsion. Third, black beauty fence post preservative and paint, or truck bed liner. Most have the teardrop sidewalls cover the floor end pieces.
End pieces can also be painted with Titebond II glue as a sealant. Tight bond II glue has very good water resistance characteristics, even better than titebond III.
3. How does the coating hold up with the exterior grade (usually pre-treated) plywood?
The key is to seal it against water. Pick your method and go for it. The better the seal, the longer lasting the teardrop... 5 years, 10, 20, 50...
4. I have to biscuit join three pieces of paneling to get to my 5x10 floor measurement. How well does exterior grade paneling work with this type of joinery?
I biscut joined mine. 1/2 inch deep biscuit cuts, 1 inch wide by 1/4 inch thick biscuit, Titebond II glued and clamped overnight. Solid, especially after glued and screwed to the lower frame.
5. I am thinking of going with regular plywood (since exterior grade is usually warped). Coating just the one side with roof tar, and the edges with an epoxy or varnish to seal them.
Perfect, long lasting, quality products.