Under the furnace on the bathroom wall, you will find a small access panel held in place with 4 screws. The drain goes into a trap, then over to the outside wall where the black tank vent pipe.
You can also work on it from the opening in the bottom of the camper.
Instead of trying to fit a Y in the pipe, I just capped the trap to seal the black tank, then I adapted the drain to a small 1/2" hose barb. Connected a hose and ran it to the back of the camper with a couple feet extra. At the end is a ball valve so it doesn't drain until I'm ready. When we take a shower, we just catch the water a bucket. Then we have the choice of dumping it back into the black tank, the gray tank, or campground disposal area.
Works for us because the TC shower has been our backup. we usually stop in a campground with decent showers ever third or fourth day and use it.