Replace Black Water Valve with full holding tank

Well.......

To replace valve you have to remove it. Once you loosen the four bolts that hold valve in place, STUFF is going to flow out.

Contact a septic tank service company and have them suck tank as empty as possible via toilet OR possibly suck it empty via a sump pump with an extended suction line down in tank.

Otherwise you will have to bucket brigade it........wear old clothes or slicker