Joe...the unnamed breaker SHOULD be the water heater. Looks like you have room there to add an additional breaker for a charger.
The reason I suggested going to the battery switch for NOW is that I assume for your future needs you will want to simply wire ONLY to the house bank which requires a charger with ONLY one output. MOST chargers have only one output and it is WRONG to buy one with more outputs than you need especially since many multiple output units will DIVIDE the total amps between the outputs so that one set of batteries will never get the full amperage you paid for. A 20 amp dual output charger may thus only give 10 amps max to each of two banks. Not what YOU want or need.
Thus...if you get a single output unit for your future needs...you must hook it up temporarily to the battery switch to be able to charge both batts you now have until you get the echo charge.
No longer posting. Reach me by PM!