digital volt meter

Depending on your monitor panel, you may be able to connect to the wires for the existing battery meter portion. That's what I did and mounted a small cigarette lighter sized digital voltmeter that reads when the monitor panel switch is pressed. No extra push-to-read switch was required.

I doubt you could disconnect without killing the tank level indicators, I would just tie on to the 12 volt line going to that panel.

Suggest you put a momentary push on switch in line with it so you dont create another parasitic draw while it is in storage.

How would you mount it on the tank display panel?

I've mounted similar ones by using a standard single gang blank outlet box cover plate and cutting a square hole using a utility knife (drill a few holes to get the cuts started). A nylon cover plate is softer and easier to cut a hole in it. Then you can just screw the plate directly to a wall - you don't need a box behind it because it's low voltage.

You could mount the meter anywhere as long as you have unswitched 12 VDC nearby. The wires to those holding tank/battery displays are pretty tiny gauge and perhaps could result in lower voltage than you actually have at the converter panel.

I'd like to mount it in the same vicinity. Thought maybe I could use existing wires or tap in?

Where do you want to mount it?

Why disconnect....you can just add the new voltmeter.