Let's go back to the original question and ask what the poster hopes to accomplish. But any sort of sealed system means that nature's scavengers can't get at the body. That slows the process down markedly. A septic system would probably take longer than just leaving the body outside.
On the other hand, using an outhouse is a classic solution to a body. The pit can be rather deep, and nobody is going to want to go looking. Basically the pit is a cesspit. Cesspits, depending upon local regulations, may or may not have to be pumped out. Septic tanks are pumped every few years, and the bones would not decompose. The body may or may not decompose in a septic tank.
A containment tank using lye or acids can accomplish destruction of a body in about 50 hours or less, especially if combined with heat. Lye is better than acids as it tends to be faster and less regulated. But you won't end up with much, if any, of a skeleton. Everything turns into sort of a mush.
So the question is whether the idea is to hide the body or destroy the body. And then the question becomes how far you're willing to go to destroy the body.
Jim