We have a significantly blocked gray water tank. Think the previous owners lived in the unit and didn't properly use/service the tank. I thought I had completely drained the unit before winter layup and that my sensors were just clogged (showing full tank), but apparently went through the winter with a full tank. Virtually nothing comes out when I open the drain valve. Since the valve and pipe are fairly small (compared to the BW), I can't get any kind of flush stick up the pipe. I have been able to get about 15' of a snake up there, and a water hose (but only about a foot because of a pipe bend), and what comes out appears to be pieces of a chalky white substance, about 1/2 to 1 inch in size. When touched, it crumbles apart. I'm thinking that enough comes loose that it starts to drain, but clogs the pipe until I run the water back through it and them some more comes out before clogging again. It doesn't look like grease; I'm baffled as to what it is. Does anyone have any suggestions on how to break it up or emulsify it with some sort of chemical that won't hurt the tank or drain pipe/valve???