'Car' will MAINTAIN a trailer battery that is already charged....provided you have a 7 pin trailer plug and charge line is hot.
But vehicle charge wire is small gauge, long and is dependent on vehicle charge system
Trailer has parasitic drains on the DC system
Hardwired LP detector, radio memory etc.
Trailer battery just sitting in storage will drain down within couple weeks.
This is NOT good for battery life
Need to get that battery charged back up now that it is run down.....then if possible plug trailer into AC power and keep battery charged via trailer converter/charger system. Otherwise.remove battery and put on portable charger.