connecting grey tanks to get more capacity for the shower

I agree with those who suggested the twist-on valve on the outlet. As long as you have one common outlet for all the tanks, this is simple and so convenient. I can share between tanks, any tanks I choose, I can use gray water to back flush my black tank at dump time, and that extra valve between blissful dumps and catastrophic dumps, is a nice peace of mind. They should be standard equipment in my opinion.