I'm a big fan of making any flat roof a 'deck'.
For the life of me, I never understood a flat roofed house like you see in CA that didn't have a deck on the roof.
Given the price of living space, and the fact that a deck(planks spaced maybe 1/4" on beams) would also insulate from sun, I don't see why it wouldn't be the first thing to do.
Maybe building codes, but I don't think so.
First thing I'd want on any RV(house-car to Brits) is a deck on the roof, even if it means adding fold down handrails.