We travel full time in our TT. Originally it had a plastic tub which developed a crack in the floor of the tub after 3 years. We replaced with what we thought was an “upgraded” fiberglass/composite ...
I would assume that the tub is flexing and eventually cracking. Maybe as suggested try to reinforce the base that the tub sits on.
Mine has short legs under a piece of plywood that the tub sits on. If yours is the same, maybe you can double up the sheets of plywood to further reduce flex.