Designing Kitchens and Bathrooms - Acrylic Tub Misinstalled, no support

Hey all,
We bought a house a couple years back and one recouring problem is eating at me, the master bath tub. Evidently from the way the tub sags in the center and has cracked they didnt put mortar or plaster under it when installing. I've repaired the crack once using a fiberglass repair kit and its holding up fine but today a new crack appeared. The tub is part of a one piece tub/surround and I just dont have the surplus cash on hand to replace it at the moment. my question is this. Since I will be patching the tub again for now, is there any ways that I can install the reinforcement now? Drill a hole and pump/pour it in? Any suggestions?

Is there any way you can gain access to the other side of the tub, even if it means a little sheetrock work in the adjoining room? If so, you may be able to get to the full length of the tub and inject some thinset or other medium to prevent it from flexing too much.

Expanding urethane foam might be easier to inject under the tub, and will cushion things. Not all tubs requires a bed, but it certainly sounds like yours does.