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.
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Best Answer
It's not a great picture, but it looks like the tub is supported by wood and foam. It's possible that the wood has rotted, or the structure that supports the wood has rotted or been compromised in some way.
You can spray foam under there wildly, if you want. But if the structure that will support the foam (and tub) is compromised, it's not going to do you much good.
You're going to want to get better access, and evaluate the structure. The answer may be as simple as adding a few extra braces, or as complex as ripping it all out and replacing it. Without a better look, it's impossible to say for sure.
Since this is a mobile home, it's not likely this is a botched installation. Instead it's likely due to age, wear and tear, and/or rot.