Measure across the outer 2 6V battery posts and if they are 11V+ they are connected correctly. ie You have a 12V battery. Get it charged with say a auto battery charger. You need to find out why they are not being charged by your house charger. Note the charger may/may not charge the engine battery depending upon how it was set up or modified.
Also start the engine, first charging the chassis battery if needed. The alternator should charge both batteries raising the voltage on each to 13-14V, just like a car. This is a test as it can take several hours of driving to fully charge discharged batteries.
Until BOTH battery banks are being charged/maintained correctly you will likely not be a happy camper.