Couple of 12 volt car batteries I purchased over the last year were well over $120, GC2 6 volt batteries can be purchased for less at Samsclub or Costco. Did the two 12 volt battery set-up every time we got a brand new RV since dealer would always installed a 12 volt, we would have them add another battery since we do lots of RVing off the power grid.
Was never satisfied with their performance, would trade them out for a pair of 6 volt golfcart batteries from Samsclub after a year. Current pair are 5 years old and doing great, paid about $130 for the set. We use more gas on a single RV trip than what I pay for at least 8 years of reliable 12 volt reserves, proper charging and watering is paramount.