if the tow vehicle was factory wired, it could have a diode to protect the battery from draining when hooked up to the trailer. If the diode failed it could give weird readings. Take the reading with the vehicle running and again when not running, and compare.
take it with the trailer unplugged from the tow, and then with the trailer hooked up.
After a weekend of boondocking
it takes me a good 3-4 hour tow to charge the trailer battery.