Yellow, discoloured shower walls

Help! Why have my shower/toilet walls gone a yellow mustard colour? My van has been covered from the end of the October long weekend. I went inside yesterday and to my shock I found the walls were all yellow in places. It looks like someone has sprayed mustard on parts. I have tried bleach and other cleaning but it won't come off.
It feels as if it's in the material or is burnt in. Any info is appreciated.

Dougy, NSW