I tapped a street 90 into the side of the trap. Then I installed a ball valve. If I open the valve the water runs out to a garden hose and beyound. Close the valve and it goes back into the black tank. Once I made the mod I think I only closed it one time. There is no y or other plumbing on the 815. The shower dumps directly into the BLACK tank. I did all my work through the little access panel Kohldad refers to. You need to be carefull that trap is not a common part. Its actually a Lance part #. The only place I could find one was from Lance. I went all over Elkhart In looking no luck.
Good luck