I found this to work the best.
First - Understand how the idiot lights work. The 4 lights are an attempt to give you some idea how much junk there is in the tank. ALL types (Gray, Black, and Fresh) work the same. The Monitor bottom light (the one marked "empty" is ALWAYS ON. It gets power directly from the 12VDC coming from the power converter and really should be marked "POWER" not empty.
0910091217 pictures from travel photos on webshots
The stud on the left side "ground" and the three on the right indicate:
Bottom stud - Should be marked "NOT EMPTY" (the 1/3 Full light) From its position on the tank one or two flushes or a short burst of water from the tap will turn on this light. This can be on if anything is in the tank depending on if your tank is level or not
Middle stud - (the 2/3 Full light) should come on about half full. Gives you some idea that the tank is working correctly and "filling."
Top Stud - (the "Full" light) indicates about 2/3 full. NEVER dump until this light comes on. (Even with Turbo flush) You MUST have a full flow of water to make sure solids and any undisolved paper gets flushed out.
Turbo flush sprays against the far wall and may not remove built up crud on the sensors if its already junked up.
If you must dump before "FULL" then fill the Black tank with water till the FULL light comes on; Then Dump. ALWAYS leave the BLACK handle CLOSED (even in full hookups) until the FULL light comes on.
To Clean your BLACK tank if its already crudded up (I do this every other camping trip for maintenance):
DUMP TANK AND FLUSH with turbo wand (through John) or turbo flush fitting. CLOSE black water dump valve.
Add 2 capfuls of CALGON water SOFTENER and one capful of laundry detergent and at LEAST three gallons of water. THEN Right before you pull out for home, or your campground, add one or two BAGS of ICE CUBES or Crushed Ice through the John into the Black Tank.
Drive for at least 30 minutes.
When the tank NEXT indicates full (Even if its your next camping trip) repeat the entire thing.
The first time you do this (if you have a clear elbow on your dump connection) you will see VERY BLACK water coming out. (Not brown like now). That is the buildup from the tank walls.