I have a fiberglass bathtub in my mobile home that is roughly 5' x 4' in size. The mobile home is a 1978 doublewide, and the tub was in the home when we purchased it, 5 years ago, so I'm sure it's very old.
There is a creaking in the tub that has gotten much worse over time. When you walk around in the tub, you can feel more give in some areas and the creaking is horrible. We do not have the money to replace the tub.
I have a small access panel in the adjoining closet, and I took a picture (attached) that shows the underside of the tub. I liked the idea of spraying foam underneath the tub, but as you can see from the picture, it is very far off the ground and there are drain pipes that are under one part of the tub.