A device that I have found to have many uses is the "KILL A WATT". You just plug it in to any outlet to get voltage. It also has many other uses including the capability to get a measure of consumption for individual services.
It has been a few years ago, but Sears Auto stores used to carry a unit to olug in your cigar lighter to read DC volts. It is very accurate, as it matched my Victron display to the 10th. The digital+lights display pivots to get a good viewing angle. WOULD PROBABLY WORK WELL IN THE COCKPIT outlet.