ALot of times the deep cycle batteries will not recover well if discharged below 50% charge state which is around 12.0VDC.
Connecting the trailer back up to 120VAC from your garage using a RV30A-15A long adapter from WALMART will power up your internal converter/charger to re-charge your batteries. You may have to turn off a few high current appliances when plugged into a house 15-20AMP receptacle.
Again if the batteries are totally discharged this may not work for you as a whole lot of DC current will be demanded from the converter/charger trying to re-charge discharged batteries. If this happens then your only recourse is to use a smaller battery charger and charge at a lower DC CURRENT level for a longer time to start bringing the batteries back to life.
Since they have been discharged like this you may find that the batteries will NOT perform at their peak performance like before and eventually will need to be replaced.
Got to respect the battery system to keep them workig at their peak performance for long periods of time. Battery replacement doesn't come cheap...
Just my thoughts here...
Roy Ken