time2roll wrote:
Rent a camera to go up the pipe.
Or buy a pipe inspection camera off ebay for under $20.
Is this a single grey tank that serves kitchen sink and bathroom? Normally air gets draw into the tank through the roof as the tank is drained. If the roof vent pipe is blocked, I would think the water in the P-traps would get sucked out? You could disconnect one or more P-traps to let air into the tank as it drains and see if anything changes. Could also add some water to the tank and pour some food coloring down the vent and see if it shows up in the tank when you drain it. I think just because you can't feel a disc cutout doesn't mean it isn't there and getting pushed back with your hand. What if it's a sewer rat in there?? :E
You could try some Liquid Drano (our KZ owner's manual says to use it), add some water and go for a drive to let the tank slosh around. It's safe on metal and plastic. We've used it a few times and gets a tank back to factory fresh & clean condition.