I had a similar problem on an airstream travel trailer we bought when it was 3 years old.
Crack was there when we bought it and was maybe 3" long in teh corner of the one piece fibreglass shower.
All I did was to stop drill the crack to hopefully prevent it from progressing and then o sealed the drill hole and also the crack (that was really not open) with a bead of clear flexible sealant.
It has stayed exactly that way for the last 9 years that we have used the trailer. I thought that with any other method of repair I would run the risk of just making it look worse rather than improving it!