On our old 2000 Four Winds, the support under the basin was, shall I say, very poorly designed and installed. I had to remove the shower walls and the basin to gain access to the area, so I installed new framing and a large piece of 1/2" plywood at the proper grade, then drilled a hole sufficient for the drain, then reinstalled & sealed the entire structure.
Original base:

Yes, that was all that was under the shower base:

New framing:

We used that unit for 5 years w/o an issue. I liked that shower... it's 1 1/2 times the size of our current shower in the Sunseeker.
Bottom line, to get sufficient support, you'll probably need to go to the effort of removing the liner and base to access the support structure (or lack thereof), and properly rebuild it.