The problem is that the 6v are usually larger than the 12v that come with a C. My RV has the battery under the step as well and the holder is wide enough for 2 group 27's but I can not put in 6v batts because they are taller and will short to the step when closed. You may have to measure the space and look for the largest single batt that will fit, change you lights to LED.
Of course if you have big bucks to spend, they now make lith ion batts the size of car batteries with more capacity but you could buy a Honda generator for less.