I installed a plastic 55 gal drum with holes drilled in the sides starting about 15” from the bottom to the top. I sat the drum in a hole about 2’ larger in diameter than the drum. I lined the holes sides with plastic drain field mesh followed by filling the sides and top with 57 stone. Has worked as a separate tank washout and septic for family overflow from campers going on 12 years. Now it will not hold you dumping all your tanks at once but would hold a weeks worth of black water with no problem while letting the grays stay open all the time.