I was just thinking about this the other day. I went to a big RV show in Orlando and noticed a few smaller rigs that had either a sunken shower pan or a raised bubble for your head. The "head bubble" seemed weird... like showering in a space station. If I had to pick between raising a roof though, or lowering a shower pan and adding a pump... I'd go with a pump.
I imagine you could cut out the opening, weld in a steel "cage" to support the pan, then just frame it in with wood and drop in the pan. If that doesn't work, cover the hole with 1/2 plexiglass and you have a floor-light! GENIUS!