The RV30A-15A "dogbone" 18-ich long type adapter looks similar to this one from WALMART
DO NOT get the small black round adapters as they will start getting hot on you after a few time being used...
Since your trailer is an older trailer and most likely does not have a smart mode technology converter/charger installed I would always check the battery fluids at least once a month to make sure they are not boiling out fluids. Using 13.6VDC constantly on the batteries for a long time may start boiling out the fluids. It is ok to use the single mode converter/chargers but you have to keep a constant check on the battery fluids and keep them topped off.
Having a BATTERY DISCONNECT is a great idea. many are available everywhere... Here is a link to GOGGLE IMAGES showing the many battery disconnect switches available. Click on the one that catches your eye.
This one is very popular amoung the RV'ers... Folks like to install this model in the side of the battery box to keep the "HOT" terminals away from things...
Battery Disconnect Switches