I changed from two 12 volt parallel wired batteries to 2 six volt series wired golf cart batteries in my motorhome. My run time almost doubled.
This was done on recommendation of an Interstate technical rep at an RV rally. The price was similar for 12 vs 6 volt. The specs on the 6 volt show a considerably higher reserve capacity. The 12 volt on board charger had 2 leads. The positive went on the positive of one battery and the negative of 2nd battery.it's basically just a really big 12 volt battery when wired in series.