i too was suprized by the condition of the plug.
the bleach must have affected it, it seemed like a little different type of plastic. or rubber perhaps.
the good news is, that it is 1 1/2" pipe thread. should be able to find a replacement plug made of p.v.c. at any good hardware store.
unless all of your plumbing up north of the border is metric!
i am the second owner of my trailer, it was in the family. i was at the dealer the day it was delivered.