I agree with Turner. Open the black valve and have someone hold down on toilet flush pedal to see if it drains out, close black and open grey and check for drainage, this will tell you for sure where the toilet is draining. I would also do the same thing for a faucet, opening one valve at a time and see what is draining where. Some newby idiot could have just gotten the valves backwards. When you hold down the flush pedal you should be able to look straight down in to the black tank.
Dave & Myra
One day your life will flash before your eyes,
make sure it's worth watching!