I made two other devices for the safe operation of my electrical system. I used one to monitor the battery through the DC circuits in the Zephyr. I used the other to correct an occasional problem with AC shore power connections.
The first device is pictured below at the top of the photo. I attached a 12 volt DC plug to the end of a set of spare leads for a multimeter. I was careful to match the polarity of the plug to the polarity of the DC outlets in the Zephyr. If this was reversed, then the meter would read negative voltage. I plugged this into any 12 volt outlet in the Zephyr to read the state-of-charge in the battery.
In some campgrounds, the line and the load side of the AC circuit are mistakenly reversed in the shore power outlets. This condition isn't safe, even though a simple device, like a light, will work if plugged into such an outlet. Other electronic devices, like my AC-DC power converter and battery charger, will not work and can be damaged if plugged into an outlet that is wired incorrectly.
To guard against this, I bought a simple plug-in polarity tester. With the tester, I could verify correct polarity in the AC shore power outlets at campgrounds. The pattern and color of the LED display on the plug-in tester indicated correct polarity and a safe outlet or reverse polarity. I made the device shown in the bottom of the above photo to use when the tester showed the polarity was reversed. It consisted of female and male three-prong, grounded plugs connected via six inches of suitable wire. I purposefully switched the line/load polarity in this plug. I made sure that the continuity of the grounded leg (green wire) remained. When I found reversed polarity in the shore power outlet at a campsite, I plugged in my adapter at the shore power location. The adaptor switched the AC current back to the correct polarity. It was a temporary fix and I used it with care. I made it very short and marked it with unmistakable colors. I did this so I wouldn't use it by mistake when not needed and cause a dangerous problem.