We recently had trouble with our CO/LP alarm while camping without a shore tie that turned out to be low voltage ( Emergency CO/LP Alarm Cut Off Switch
). Our a-frame did not come with a voltmeter so we had no idea what the actual voltage of the battery was until we got home (I brought some tools, but no multimeter). I had planned to install a voltmeter even more we had the alarm issue, and had ordered the meter and momentary switch (from E-bay). Total cost including shipping was less than $6. I installed it today using the wiring harness behind the paneling at the entrance of the camper.
It's hard to see in the photo, but with the momentary switch pressed it shows 13.6 volts dc. It's plenty bright enough, even in daylight.