Your assessment of the materials seems right on. Building RV interiors is not like building a house. 2x10's... really??!!! Your could easily overload the axle
or even the frame itself. It'd be interesting to weigh it now as a reference point.
On the other hand, be careful of too "open" a plan. Most single-wall molded fiberglass trailers require some floor-to-ceiling cabinets or bulkheads to support the shell. Structural panels with 1x framing and a thin skin on both sides provide support without weight
Since it looks like you live in the snow belt, make sure you maintain some support to the roof during your renovations.
Best wishes getting this gem set up the way you want!!