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Mineral oil will kill the good bacteria in the tank, don't use it.
"Cesspool" additives, enzymes to help decomposition, are a good idea. Way cheaper than the marine versions of the same stuff. But with good ventilation and keeping the toxins (bleach and oil) out of the tank, it should build up the right bacteria balance to keep itself in good shape. The tank MUST be well ventilated to work properly, and if you can run two vent lines to different places ("flow-through ventilation") that wil keep it odorless. One good vent line can do the same thing, just not quite as reliably.
If that valve really is seized and you can't free it up, just replace it, in the long run that's probably simpler & cheaper & faster.
One thing that will help the tank is your choice of toilet paper. Some brands are made from finer softer pulp and they dissolve faster, and that's what you need. (Offhand, I think Charmin is one of them in the US.) If you take one sheet of toilet paper and place it in a bowl of water, and it dissolves on its own, that's good. If you come back and it is still a solid sheet--that's bad. Whatever meets your needs and dissolves fastest, that's best for a holding tank.
Last edited by hellosailor; 08-31-2006 at 11:34 AM.