The first thing I would do is drill 1/16 inch holes just beyond the end of each crack which should stop it from propagating further. Fill the holes with some kind of epoxy eg. Waterweld by JB weld. This is amazing stuff and might help repair the cracks also.
Then I would do a Google search for `tub surround for rv' or some such phrasing and use that info plus responses to this post to decide what to do.
Dick_B
2003 SunnyBrook 27FKS
2011 3/4 T Chevrolet Suburban
Equal-i-zer Hitch
One wife, two electric bikes (both Currie Tech Path+ models)