Go to Harbor Freight and buy a $4.99 (on sale) Trickel charger, connect the alligator clips to the respective engine battery treminals, run the wire from the battery box to the nearest 110 outlet and plug it in.
DONE.
I have one installed in my unit and I ALWAYS have a fully charged battery. I have been in Florida all winter and my battery is always fully charged.
When I pull the shore line, battery charger is disconnected but waiting for the next time I am hooked into 110 supply.
Nothing can be easier than this.
I also use these for my cars and motorcycle batteries that are in storage when I am in Florida in the winter.
Battery is never overcharged or run dry.
That's what these chargers are made for.................................................