I believe you have some very serious problems here. The house should not be over the septic tank in the first place. Do you have traps on the drains? The air admittance vents, aka Studor, can not be the only drain in a system. There must be at least one vent to the outside. Most plumber shy away from them anyway because they are mechanical devices that can/will fail. I would strongly recommend you contact a licensed plumber to assess your problems. Do not call Grandpa.