Another tip that you might like that will help keep from filling your grey tank to quickly, when running the water in the shower to get just the right temp run the water into a bucket. You can then use that water to flush the toilet do dishes or whatever.
You can also but a shutoff before the shower head, then once you have the temp you like use the shutoff instead of the faucet. Then you don't waste water finding the temp you like again. Here's the one I installed from Amazon, it works like a champ KES K1140B Shower Head Shut-Off Valve Solid Brass, Chrome - - Amazon.com