Yes...I mounted two 6 volt deep cycle golf cart batteries enclosed in plastic battery boxes. Since you already have a 12v battery, try to add another of similar age and capacity to prevent one from leaching voltage from the other.
Adds a bit of weight to the tongue but works well for an extra couple of days dry camping. I also added a battery disconnect switch to help isolate the batteries while not using the camper.
2015 Rockwood A128S
2010 Rockwood 2604
2014 Ford F-250 CC SB 2WD
2008 Toyota Sequoia 2WD