My campers battery compartment would not fit two group 27 batteries (like 1/4" short), and two group 24's have a low amp hour capability, so I went with two 6 volt batteries that just fit height wise and had room to spare sideways. 95% of my camping is remote Alaska dry camping so I needed battery AH capacity to last long enough for long weekends. Dual 6 volts give superior amp hour capacity and allow me to go up to a week dry camping. I've never had to add water in over 5 seasons of use either, they are great.