As far as I know it should be a sealed system between the drains and the tank. Only thing open to atmosphere should be the vent pipe out the roof. I really don't know what the connection to the top of the tank looks like. The tank is polypropylene I think, and the pipe is ABS or PVC. As far as I know they can't be joined with glue so probably a coupler of some description. Doesn't sound like fun to get to. Post pics if you figure it out. Sorry I can't be more help.
"It`s not important that you know all the answers, it`s only important to know where to get all the answers" Arone Kleamyck
"...An unarmed man can only flee from evil, and evil is not overcome by fleeing from it." Col. Jeff Cooper
Sunset Creek 298 BH