On board battery monitor

Steve,

Mine is very simple, a three hole mount with a faceplate. Has a LED voltage readout, a cigarette lighter plug and a USB port.

from amazon for $14.  i have  direct wired 12v adapter.

How important is it to have a direct 12v connection to my panel?  For example, I've installed three 12v USB/cigarette units, but have tied in off of two lights and the stereo.  Will I get an accurate reading off my existing ports units?

 A plug in style shown above would only consume during use. My panel mount is meant to be hardwired, but of course I was wise enough to put a momentary push button switch inline with the meter leads so it only uses power when I keep my finger on the button for a few seconds. And voltmeters use almost NO electricity to take  measurement so no big worries about it hurting battery capacity. Their energy use is measured in milliamps.

The most accurate measurement is made right at the battery terminals. The further away from the battery the less accurate depending on the wire size and length and the current being drawn at the point the tap is made. The length of wire from the tap point to the voltmeter is not important. That says it is best to go all the way to the battery, not any circuit from the power panel and not even the battery connections at the back of the power panel. That said the question becomes how accurate is good enough. Tapping into a lighting or other circuit is OK IF nothing on that circuit is drawing any current. That means lights/fans/radio/booster/ etc. are off. Nothing other than a voltmeter plugged into the USB or cigarette lighter sockets. It's also necessary to have the battery at "rest" for several hours before making the measurement. Rest means nothing drawing current except for the residual from the propane/CO monitor. With all that said you can get a general idea of battery condition by watching the VM connected to a branch circuit over time, but the final word on battery state of charge will have to be made at the battery after a period of rest. A reasonable compromise can be had by connecting the VM to an unused fuse on the power panel with nothing else on that circuit. Nobody said this was easy, but anything is better than the 4 LED idiot panel supplied OEM.