I have a couple of voltmeters monitoring our coach batteries all the time, and they're rarely unplugged. There is no power draw to be concerned with from running digital voltmeters constantly.
One voltmeter is easy to read back in the coach area, and one voltmeter is right on the dash so I can see voltage in the coach area while driving. The reasons to watch coach battery circuit voltage while driving is to indirectly see what the engine alternator is doing (since it's feeding the coach system when the engine is running) and to make sure I've switched the coach batteries back into the system after gassing up so a I know that the refrigerator's circuit board is getting power (I switch the coach batteries off when gassing up to insure that there is no open flame in the back of the refrigerator near the coach's gasoline fill spout).
The voltmeters I use read to four digits so that I can monitor voltage in 1/100 volt increments, instead of the usual three digit voltmeter's 1/10 volt increments. I like to see the voltmeters drop 1/100 volt with each LED light that we turn on. ;)
I also have a coach battery ammeter on the dash so that I can see how much current is going in-to (charging) or out-of (draining) the coach batteries at all times while in storage, camped, or driving.
