By any chance were you operating a refrigerator
on 12 Volts? Many automobile alternators entire output may be used by the refer and there might not be any power left to charge the batteries.
If that appears to be your case, the cure is to purchase a heavier duty alternator.
They may be relatively expensive, but if you knew exactly what you were looking for, it might be possible to locate what you need at a wrecking yard.
Some other things to check---make sure the ground connections on both trailer and tug are clean and tight. That step alone cures more electrical
woes than any other! And make sure there is water in the battery.
& Ann K.