Are my dump valves mislabeled?


2021 Milestone 377MB, we picked her up at the dealership a month ago, first time RV owner. I have her parked on private, family-owned property. We've been dry camping for the last few days as a sort of trial to learn her and work out the kinks. She has two grey tanks and two black tanks. The rear galley grey tank has its own termination valve that I've left open with no issues. I treated both black tanks with Thetford liquid black tank treatment on day one. This becomes relevant in a minute. The remaining three tanks each have their own dump valves into a common termination drain. From front to back, they are labeled Waste, Grey#1, Waste. Since getting the coach home, I have never opened either black tank dump valves, but I have made sure the valve handles are pushed all the way in. We've primarily used the forward toilet only because the rear toilet is in a very tight space. The forward black tank gauge is already showing full. I learned from this forum that those gauges are notoriously unreliable. I've not experienced any "burping" of the toilet yet. A family friend operates a local honey dipper service, and they are coming out in 2 days to empty the black tanks.

Here's where it gets interesting. The forward grey tank backed up into the shower this morning. No problem, I'll just head outside and crack the forward grey valve. I first checked to make sure the black tank valves were still closed, and slowly opened the forward grey valve. Sure enough, the shower began to drain. BUT, the water coming out of the grey tank had a blue/green color like the tank treatment, and smelled like tank treatment and poo with bits of toilet paper but no solids. I closed the valve and went back to the panel. The forward grey tank that was quite obviously full now reads 2/3, and the forward black tank that was reading full now reads 1/3.

My first thought was that the forward black tank dump valve could be faulty and not sealing. But if that were the case I would have black tank contents leaking out when I remove the termination cap, and I don't. Really scratching my head here. Why would the levels on both tanks go down by opening only one valve? Hit me with some wisdom!
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If you don't already have a clear adapter on the sewer outlet, you should get one.
I bought one a couple weeks ago and used it this morning when the honey dipper came out and emptied us. The valves are 100% mislabeled. Of the 4, two are correct and two are transposed. The forward black tank drained crystal clear, and the forward grey tank drained... not clear. Now I know why the grey tank smelled like tank treatment :eek:
If you don't already have a clear adapter on the sewer outlet, you should get one. Then get some food coloring and use a different color in each drain to see which drain is associated with which pull handle. Here's one from Walmart that has an external gate valve too.
I love that, I've got two grey and one black, and have mislabeling issues but didn't know how to correct it. I'm a rookie camper so advice from the pros is always helpful. Thanks and Happy Trails!