Holding tanks

I think that 2 gray tanks is often due to plumbing requirements, like the bathroom and kitchen being located far apart. Is that the case for you? I wouldn't be surprised at all to see an incorrect panel installed in an RV either.

With water in shower, run some in the other sink. If the level in shower does not rise immediately, start at that sink, and listen to drain water flow. Best not to need to find a full tank to use the sink

We have two gray tanks: the bathroom sink and shower are one and the kitchen sink is the second, but we also have two dump valves.

It's not uncommon on longer units. If the kitchen is at the back and the bathroom is at the front, the plumbing to run to one tank is a hassle. Plus, you generally fill more of the gray tank than black tank if you are on a water only site.

But based on the below, you only have 1 gray tank:
https://library.rvusa.com/brochure/denaliliterature112rev313web.pdf

You might try a second experiment. Run the kitchen sink to see if the other tank fills per the panel reading.

Most likely, they use the same control panel for all their models. Our non-slide toy hauler has a position (no switch) listed as "slide out" on the panel.

Just because there is a switch, does not mean there is another tank. My non existent 2nd galley tank shows empty when I use the buttons.

While use a panel that indicates 2 grey tanks when only one is installed.....

Because it is CHEAPER to use One Panel for all models

CHEAP/Less Inventory