I think that you will find that it is made of PVC.
PVC pipe glue will disolve a thin layer but unless you opened the crack it wouldn't be a good way to repair it.
If you could get to the back side you could glue a patch using more PVC material.
Personally I'd take it to a boat yard and ask their opinion.... and probably let them repatr it if possible.... they do PVC repairs everyday.
And... if it has been that way for a long time I'd probably just continue to watch it, a repair from the shower side will be ugly unles you glued something made of PVC that has a function (like a soap dish) right over the crack.
Question... how old is the camper?
I have found that the pthlates slowly evaporate from PVC over time.... like 10 years then it breaks... sunlight or heat accelerate the pthlate loss. (pthlates are the chemical that makes PVC flexable... the new car smell)
Good luck
Chet