What voltage does it get down to after sitting for a few days or a week with this plugged in?
I have a total of 3 of these USB outlets on this truck. 1 shows voltage, the others just have a single LED. They are all always on, because the circuit is always hot. To be honest I've never really paid attention to the voltage reading after a couple weeks of it sitting, but it's always enough for it to start.
If you were really concerned you could easily wire in a toggle switch to turn off the light, and I recently
saw a socket that has a switch built in haven't tried this one but looks like a decent alternative if you are willing to give up one USB and are concerned about the drain of the voltmeter always on
The only time I have battery issues is in spring, after it sits for a couple months. This year I'm going to mount a hardwired solar panel trickle charger/maintainer on this truck. I did that on my first gen and it worked great