If the sludge has turned to concrete
There's nothing that'll disolve it that won't damage the tank and the plumbing too. If it's still "mud," the best way to get rid of it is with a hose (NOT the same hose used to fill water tanks!) in the deck pumpout fitting--which sends water directly to the bottom of the tank...add a couple of gallons...pump out. Add a couple of gallons...pump...until the water pumped out is clean. Better yet, don't let sludge accumulate. Nominally rinse the tank the same way after EVERY pumpout or dump at sea...and give it a thorough rinse after every 5 or 6 pumps/dumps. The main cause of sludge is chemical holding tank products...they kill SOME of the odor-causing bacteria, but they're non-selective...they also kill the bacteria necessary to break down and emulsify (liquify) waste, so solids and paper can only disolve into little tiny particles that settle to the bottom of the tank, along with chemical residue, to become sludge. Organic tank products like Odorlos and K.O. not only work with the naturally occurring bacteria in waste to allow natural breakdown, but also are more effective in eliminating odor...they don't replace waste odor with another equally objectionable odor, they prevent odor from occurring in the first place.
