Grey tank overflow

If it was pre-plumbed it make not be going into the gray tank. The washer on my Sahara goes directly to the sewer outlet, bypassing the tanks. If this is the case, you may want to close the gray tank drain valve.

I would suspect a drilled plastic plug fell into the Gray tank when they built it and with the volume of water injected into the tank when the washer pump is ON, is causing this round plastic to block the exit fitting at the dump valve. Then it falls over and does not block completely. The fact that it comes UP into the shower does not point to a vent problem. The water HAS to fill up the tank to then go back UP into the shower. Having a vent block would cause the water to come back out the washer down pipe. UNLESS, the washer drain plumbing is somehow connected to the down pipe of the shower where it goes to the gray tank. The Y fitting is backwards where the Washer ABS and the Shower ABS is tied together. The force of the washer pump is pushing the water back up the one section of the Y. Doug

Moved from Forum Technical Support

I have checked the drain hose and the valve is open and water is draining except during the washer drain, sometimes.

Tank will not gravity drain w/o proper venting.

When using washer......have you visually check that water is flowing out of sewer hose?

Blocked vent line OR blocked drain line (old cut-out got stirred up off bottom of tank due to pumped water flow from washer agitating things)

I would make sure the vents aren't blocked on the roof. Take the caps off and make sure the pipes were installed so that the cap doesn't block it.
This is a common manufacturing defect.