How old is your camper? My Jayco had a similar issue. The plywood base only covered about 2/3's of the width of the shower pan, so if you stepped on that 1/3, it flexed and scared me. Turns out that the flexing actually allowed the drain to come loose and caused a leak at that connection.
Thankfully I was under warranty. They removed the shower, installed a new piece of plywood and stiffened it right up. Fixed it 100%. Without seeing actual pictures of yours, I would imagine the fix will be the same. Remove the entire shower/tub and install the support with the correct size hole.
If you have room from below, you could cut a new piece of plywood to fit the oversized hole (like a patch) and cut it in half. Slide one side in, then the other side so that the patch is flush with the floor and the plywood. Then use a larger size piece of plywood underneath the existing plywood and patch, and use a strong poly adhesive to adhere it.
That would be the only way I could think of about attacking it from below, if there was enough room.