[JRTJH]

The problem with any "remotely located" DC voltmeter is DC load. Most USB charger stations are wired off branch circuits from the main battery wiring. Most commonly through the converter/charger DC fuse panel. When using a 14 or 16 ga wire to power a 2 amp circuit, the voltage will drop based on the power consumption at the USB port. Installing a voltmeter at that point will monitor the voltage drop, not the battery voltage. So, essentially, if you install a voltmeter at the terminal end (along with the USB port) then if anything is plugged in along that wiring run, the voltage measured will be that line only, not the true battery power.

If you want to "try it and see", then once you have the voltmeter in hand, pull the USB charger port, use temporary wiring, connect the volt meter, turn it on and read the voltage (should be 12.6 +/- VDC. Then plug in something to the USB port and start charging that item. Monitor the voltage on the meter. My thoughts are it will drop to 11.0 +/- VDC. If so, then it's not reading "true battery voltage" but "voltage drop through a branch line". It will "work OK" wired that way as long as nothing is plugged into that branch line (from the fuse panel) but the way trailers are wired, that may not be the only device on that line, so reading "battery voltage" would be "iffy" .....

The "best method" is to either monitor "true battery voltage" with a dedicated line to the volt meter from the battery terminals, or to monitor "true converter applied voltage" with a volt meter connected to the input terminal on the converter/charger (the big red wire on the power center).....