[PARAPTOR]

I bet it was interesting when you took that cap off With the cap on yes all is going to back up taking the path of least resistance, that valve in the back got the gray is probably not piped into the other gray and black tank, so no issue. Now the black and other gray my guess are piped together a "Y " somewhere. So depending on piping that waste could backup into the gray.

Having the black open and depending how long that stay was you could be developing a solid pile in that black tank. What I would suggest is take the cap off attach a sewer hose and if you have one use your black tank flush system to get water in tha black tank to start flushing it out. If not you may need to get a hose and get water into the tank through the toliet.


As far as the gray I would starting to get water into it and start flushing it