I used 4 inch plastic pipe to serve as a grey water holding tank. Doesn’t hold much, but enough not to have to put a bucket out every time you brush your teeth.
I was able to use the brackets used to mount the fresh water tank as support for the plastic pipe using ordinary strapping.
I did another mod today to improve drainage of the sink. I had used small air holes in the pipe/tank originally and drainage was not great. I increased the vent to a one inch pipe nipple today and it’s much better. Not perfect because I am using only a 1/2 inch drain as part of the connection but much better than it was originally.