You can buy a deep cycle Optima battery 34 series that are smaller than most 27 series. The AMG Optima can actually be installed upside down side ways and are water proof.
I have seen rv trailers that when plugged into the car via the lighting plugs also has a charger, charging up the extra rv battery.
There are many ways to accomplish your simple needs with out going overboard. The solar charger really works well, we use one on a boat hoist to keep the boat battery charged and the hoist battery charged using a simple cig. lighter adapter.