1) Depends on how well ventilated the tank is. A well-ventilated tank shouldn't need assistance, where a poorly ventilated tank often will. The bacteria that do most of the waste processing are highly aerobic, the ones that make the stuff stink to high heaven are anaerobic.
2) Depends on the type of head you have, how often it is used, and how you flush it.
3) I've never heard of mineral oil being destructive of the seals or parts in a holding tank
. YMMV, but I don't think it is much of an issue. BTW, if anyone knows anything more definitive, please let me know
4) The best thing to do, when you're going to be pumping out the tank is to fill the bowl of the head with freshwater and pump
it dry, then repeat. Then as a final step, fill it a third time and then leave the lever in the pump
dry position. This does two things. One, it flushes the system with fresh water, so the amount of sewage and salt water in the hoses is minimized. It also helps loosen and dilute the sludge in the holding tank
. And finally, when you go to pump
out the holding tank
and attach the hose, you will know the tank is completely empty because it will empty the water from the bowl for you.
Hope this helps.