Originally Posted by WolfWhistle
The OP can install a meter anywhere but if it is battery that holds interest then it must be connected at the battery.
I think I understand the reasoning behind the disagreement now. You are envisioning measuring the current or AH from the battery - Trimetric or similar. In that case, a shunt (resistor) is placed in SERIES as close to the battery as possible to capture every bit of current flowing from the battery. The shunt is probably on the order of 0.005 ohms (must be accurately manufactured to get good readings) to keep the voltage drop across the shunt to a minimum. Even at that value shunt, it's easy to get a drop of 0.1 volts or more between converter and battery.
The OP is installing a voltmeter inside or outside her camper - which is a pop-up camper with limited 12V loading. The voltmeter, which is placed in parallel, not series, with the rest of the camper, has an internal resistance of 100K or better (usually 10M or better). The voltmeter's resistance - in parallel - would reduce the overall camper resistance in the 6th decimal place at worst. It's not going to be measurable or noticed.
If there is no shunt - OP was talking voltmeter, not Trimetric - then there is only the voltage drop in the camper wiring to make the voltage inside the camper different from the battery. A pop-up camper has minimal loads and minimal wire lengths compared to a 35ft travel trailer. You have a heater fan (4-5 amps) if in use, 2 inside LED lights (0.8 amps) if in use, an automotive stereo (unknown) if in use. The possible CPAP and heater use will be the only loads of concern, and the #8 wiring from battery to converter is not going to have any significant drop with 10 amps going through its 6-8ft length.
Hence my statement that it doesn't matter where the voltmeter is mounted or tapped in. It should be where ever the OP finds a mounting spot that is easy to read when she wants to check the battery voltage.
When camping with a CPAP, the recommendations are to wire into a 12V circuit like a cigarette lighter. Run the CPAP with the humidifying off. That will give you about 4 nights (no heater) of CPAP use without recharging and not running the dual golf cart batteries below 50%.
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time
trip to the Utah 5 begins Saturday