I have the same setup with my shower and I questioned the dealer about it when we picked up the camper originally. They indicated that this was standard and that the shower pan extended far enough up the wall behind the surround to keep it from leaking.
Fast forward 6 or so months and the shower started leaking on one of our trips. I took it back to the dealer and they caulked the gap that I was originally concerned with. They thought that was the source of the leaking.
All seemed well until this last weekend when it started leaking again!
I'm not sure if the leak is coming from somewhere where the gap existed, but I bet you one thing...I'll be doing the inspecting and caulking this time rather than wait around for at least a couple weeks for them to get it repaired.