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Outside on the driver's side of the unit. Some of these are in a compartment and most are visible under the edge of the trailer. You should have some labels on the lower side that would say "Black" or "Gray" and this is a gray water valve. Some of these larger units have 3 valves............2 gray water valves. One may NOT be where the others are and you may see it behind a tire and kind of out of sight. It should be close to straight across from the sink area. Hope that proves helpful here. Please get back to me if you need more as all follow ups are included in our fee.
You would see just the T-handle............usually gray or black plastic. Some are located inside a bay storage area.
I attached a link that shows the whole valve............you would see just the handle part. http://www.pplmotorhomes.com/parts/rv-toilets-tanks/20097.htm
They are all in that lower area along the bottom........I can't tell you exacty as we don't have plumbing diagrams for these. When in the shop we look for them just like I explained it to you. I can opt out of this if you want another tech to offer something on it........