unfortunately, two things are occurring here to provide the potential for 'leakage' under the shower: incorrectly attached/glued ptrap and piping, and a shower with a floor that flexes under weight.
The access panel, the same place on my Palazzo, is not something you 'expect' to see right above the wheels of the coach, being that water thrown by the wheels is certainly capable of being thrown thru any slight crack or unsealed area of the 'hatch'.
The access panel makes sense, but it's location is not ideal, but, with the layout of coaches like ours, with the shower placement where it is, there's little to do about it.
Now, my diesel has air bags, so my access is much easier than yours, with the air bags inflated for travel. I suspect, though, when I park and deflate the bags for leveling, I have the same issue you do - how to 'get to' the screws and access panel.
When you do eventually get access, you'll see basically the 'p trap' portion of the plumbing, because it basically has to dip down 'thru' the flooring/under the shower base, which is probably more of the reason there is an 'access hole' in the first place.
One thing to do is have someone walk in the shower while you are viewing thru the access hole. If you see very much 'flex' in the piping, or movement, chances are this is 'why' you have a drainage leak. If they run the shower water while doing so it may even leak WHILE you are watching... plumbers 'pvc' or 'pipe' glue spread as much around the connection as possible might seal it off enough ... this is not 'pressurized' water, but simply the water from the shower's drain as gravity moves it down thru the piping. It's only going to 'leak' when someone showers.
Another thought is to try to squeeze something slender under the shower base, to prevent further flexing under weight, like a long slither of 1x2 wood, etc. IF you can even fit something like that thru the hole. It's about the only option.
two Campers, two Electric cars
former diesel pusher traveler