Just a point of notice, not all units go into the grey/black tanks. Mine goes directly into the main dump line. I have a 2010 Montana 3750FL. When we first had ours installed, we were told that it went into the rear grey tank and after the first rinse cycle our bathroom floor was flooded
We too have the "P" trap in the wall, but one of the first things I added was a blade valve at the end of the sewer pipe, so I leave mine closed until we run the washer/dryer or are dumping the tanks. This also helps prevent "surprises" when first hooking up and one of the valves was cracked open just a wee bit, ask me how I know