Repairing Shower Surround Cracks

I might try using some awning repair tape (on the front), assuming it's a plastic/fiberglass sort of enclosure, particularly if the cracks are on the vertical parts and not in the pan part. I don't know how well it would work, but it seems it would be worth the attempt.

If it's the pan part, check that the support structure underneath is good. Some RVs seem to come from the factory with rather questionable shower pan support designs.