Dude, that really stinks! I can't imagine what would cause that unless the support under the tub floor was not adequate or you had a lot of weight in the tub. My only suggestion is ensure the support is adequate when you make the next repair.
If you replace with a shower, maybe you can save some $ like you're thinking. I'm sure this is frustrating.
I don't typically go looking for other "excuses" for a problem but there may be some real merit in cheap plastic. Although, old one cracked and now new one lead more towards installation problem.
FWIW I just googled rv bathtub and it looks like the standard 40" ones are about $140. Maybe another option and not as pricey?