I connected my kitchen sink to the black water tank in 2004, 28' Classic.
Quick over view of what is required. In the closet in front of the shower you will find the vent line. To get to the plumbing lines you have to remove the floor out of the closet along with the side panel that covers the shower valve. Locate the vent lines, one is for the grey water tank and one goes to the black water. I put a waste valve in the sink line so I could drain into the black water tank or into grey water tank. If you can do and understand basic plastic pipe plumbing it is not a hard job. Take your time and get the various pieces and tie into the black tank vent line--make sure to keep a positive vent in the system. Keep the drain line pointed in the right direction and there is no way for the black to back up. Even if the top of the toilet is full of Black water it can not over flow into the Kitchen sink. If that happens you have a bigger mess to deal with
During the mod I discovered a hole in the vent line that had been there since the unit was built in 2004. When they put the light switch in for the shower they drilled a hole in the vent line and covered it with duct tape...so much for quality control.
We did this mod because the grey water tank was always full way before the black and then having to add a lot of water to the black before dumping. We use more water wash dishes than using the toilet.
Every place we have stayed dumping anything on the ground is not approved. One person might get away with it but when everyone is doing it ----what a stinky mess. And driving down the road and leaking on the road is just gross.