Hi Dan,
Sounds like you've gotten on with your project. Glad you found the Forums -- and welcome! I would suggest the search function to find useful threads but it'd take some hit and miss -- I'm not sure what search words would lead this in productive directions.
My Argosy black tank was unique in being above the floor, so we may be talking apples and oranges. The grey tank below the floor has a galvanized steel box that protects the foam insulation around a plastic grey tank. I dunno...
A member who has done a lot of work with rethinking tank installation is Smokeless Joe at
this thread. Also see
here. Read the threads and maybe PM him if you want to chaw on this further.... We'd welcome your posting further questions and your progress. Glad to have you aboard!