connecting grey tanks to get more capacity for the shower

Lexx wrote:
Here's an image I got off rvtrader that shows the dump valves. While the dealer states that there are two grey tanks, this photo seems to indicate a single grey tank valve.

You can't really tell by that photo alone.
My convenience center is similar to the photo but I have another set of valves that control my rear waste tanks. Those valve are located behind the tires at the rear of the trailer
I have 2 grey and 2 black tanks. I also have to discharge pipes.