wjwheeler, Same thing happened in my 3400RL and it's located in the same place behind the shower wall where the pocket door slides into. I was able to take the shower faucet loose from the shower wall and fish the water lines to the black tank flush through the opening. I had to reach in the opening to the right and took a bit to get it through the opening but was successful. Ordered the vacuum break/check valve
HERE. If this happens again I'll just connect the two pieces of Pex tubing together and not worry about it anymore. Tim