You seem to be on the right track. If there's not a crack in the shower pan and no seams that could be open in the shower enclosure then caulking the top and making absolutely certain the the drain is seated and caulked properly would be the next things to try. I'd also caulk around the water controls as suggested above. Shouldn't have happened, but make sure the drain holes in the door holder all point to the shower and not out in the bathroom. One other thing that can happen would be if the leak happens following flushing the black tank since the backflow preventer is often in the bathroom and is known to leak on occasion.
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