Provided there is room in the tray your best "Bang for the buck" is a pair of GC-2 from Sam's club, these are, when being transported from store to RV, six volt batteries but when properly installed (wired in series) a bit of engineering magic happens and they become one BIG (size 4D to be precise) 12 volt battery.. that is 230 amp hours, for the cost of about 150 in most cases.
They are also DEEP CYCLE. this means they stand a far better chance of recovering from an "OH, ****" level discharge.. MARINE/deep cycle (most 12 volt batteries are MARINE/deep cycle) are really starting batteries and do not like a deep discharge.
Of course you do need to DISTILLED WATER GC-2s from time to time (Mine have gone 3 years w/o but I baby them)