Picked up one of these "3 way USB charger with a voltmeter"
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Charges/runs a bunch of stuff while mobile or at least in the truck. Not yet used it in the camper. Voltmeter seems to be fairly accurate BUT you CAN NOT see the little numbers in the daylight. Voltmeter active anytime the thing is plugged in.
Would likely be more visible in the trailer. Now you don't have to install a voltmeter if you have a 12 volt outlet (Which I had to install)
$10 or $11 depending on color.