Matt: you may want to consider building a giant C-clamp (to lock n the vertical tub walls before the floor is removed), with foot wall (under the camper's wings)...
...however, I really like that idea of a steel box tube frame under your existing floor (sub-frame)...the altitude addition doesn't seem bad (weight?). Just add a steel cage to capture your rear and low hung tank (or, remove that tank before you start in anticipation of a newer slim tank). Once the steel sub-frame is in place, you can lose an inch to the interior, by building UP a slim basement, or a step-up floor to accommodate the new tank & heating blanket (?) There are RV holdng tanks as slim as 5 inches H (22 gallons) available.
