The truest reading will be direct from the battery(s)
A good reading will be from the battery input to your 12v distribution center, or the output of the converter (under the fridge ?)
Any other place could vari, as it is getting farther from the batteries.
I have one volt meter on a good 40 amp circuit that runs my entertainment center, and it sometimes reads a bit lower than another volt meter at my 12v distribution center, but only when sat TV, or other device is on.
I hate connecting stuff directly to the batteries, except to a terminal block very near the batteries