Ah, yes...the dreaded sludge...
That's what you're seeing George, and it's common in holding tanks. More common in tanks in which chemical products--as opposed to organic--are used, because the chemicals kill the bacteria needed to break down and emulsify solids and paper...they can only dissolve into little tiny particles that settle to the bottom. Left alone long enough, sludge turns to concrete that cannot be removed. That's why it's very important to rinse the tank regularly--about every 4th or 5th pumpout or dumpout--to stir it up and pump it out while it's still only sludge. Since you have an inspection port, you can just stick a hose (NOT the one you use to fill your tanks!) in it to the bottom during pumpout and keep flushing till what's in the tank is clean. Without an inspection port (or one that's easily accessible), the best way to do it is by pumping out, then stick a hose in the deck fitting--'cuz that sends water into the tank at the bottom where the sludge is--and fill the tank about 1/4 full...pump that out...do it again, several times. Or, just let the water run till the tank overflows out the vent and continue to let it overflow till the water coming out the vent runs clear. Then pumpout or dumpout the water. 2-3" from the bottom is a little bit short for a pickup tube, but not bad...there's no way it can be closer than about an inch without risking a clog.
