Sounds like its not properly supported under the tub, or what ever type of support has failed or worse the sub floor has. I think you need to find a way to see whats going on under the tub. If theres no access panel, might have to make one. You can also check the under belly right under the tub location for softness etc since you had some water leak issues. keep us updated and good luck!
*Currently trailer-less but shopping
07' GMC Envoy w/ P2 controller (current TV)
88' Chevy K5 Blazer (soon-to-be TV)
74' VW Superbeetle (1303)