why separate tanks for grey and black water?

BB_TX wrote:
There has been a couple occasions when I forgot to check the grey tank level. Backed up into the shower while I was in there. Found myself standing in an inch or two of water. Glad it was grey and not a mix of black.

I believe this is the reason and it's regulation not neccessarily a real issue (assuming you drain the tank before it overfills). The govt assumes someone will overfil and it's a sanitation issue then.

Since home built trailers don't have to be inspected (as far as I know), I doubt anyone will come after you if only put in one tank.