These are pretty neat to have if you have a 12VDC cigarette lighter plug. I don't have any in my trailer except at the OTA Antenna Panel. I am not a fan of using the cheap 12V cigarette lighter plugs...
I hard wired a three digital meter panel in my setup - A DC VOLTMETER for each battery bank and a DC Current meter for the main DC line coming from the batteries.
ALot of folks really like the Bogart TRIMETRIC battery monitor setup TM-2025-RV...
When we are camping off the power the power having a good BATTERY MONITOR PANEL is a must thing thing to have for us. When the BATTERIES get down to 12.0VDC it's time to start thinking about getting them recharged back up otherwise you will start doing damage to your batteries.
Just my thoughts
Roy Ken