To expand on the previous post. Multimeter from Harbor Freight:
https://www.harborfreight.com/7-func...ter-63759.html
$7. If your batteries are charging from the trailer, the voltage between the positive and negative connections to the trailer at the battery should be greater than 13 volts. If the voltage is less than 13 volts, the battery is not being charged.
Most trailers the battery disconnect switch on/off indicates if the battery is "on" or "off". On means the battery is connected to the trailer and will be charged by the converter when plugged in - or will be supply trailer loads such as water pump and lights when not plugged into shore power. Off means the battery should be disconnected from the converter and the main loads in the trailer. However things such as the propane/CO alarm and emergency breakaway brake switch will still be connected and the battery will still be discharging.
You can disconnect all cables from the negative side of the battery itself, and then the battery will just self discharge internally a small amount but will last for 6 months on a full charge.