We have the same rig and love it. Just finished a 1300 mile shake down tour and now putting my fixit list together for the dealer. So far it's a pretty small list.
And to your question, I have 2 grey tanks and 1 black. The grey tank valve furthest back under the bed slide is for the clothes washer, if installed. Further forward closers to the sewer drain are the other valves, the one furthest forward is my grey tank valve that drains the kitchen, bathroom sink and shower. The valve just behind that one is the black tank drain valve. If you look closely, you can see that the valve handles are colored either grey or black for their respective tanks.
Hope this helps and feel free to ask of clarification if needed.
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