Shower drain CS-S

captainwings

Aloha, Sink water from kitchen sink backs up into shower. Gray water tank empty and vent is clear. I'm thinking sink drain must connect to shower drain BEFORE drain trap. Had dear wife pour a gallon of water down kitchen sink and I observed a burp of air followed by sink water coming up from shower drain. Any thoughts on how to clean out trap?

Nomad

You can get at the trap under the camper - it's a black looking box. Have never open it up, just read it on the forum some time ago.