First, unless you physically damaged the battery and it's less then a year old, you need to replace both. They wear like front tires, always in pairs.
Yes you can replace with one AGM 12 volt battery but you will have about 1/2 the capacity of the pair of 6 volt batteries. Thats fine if you don't dry camp and only use battery for lunch and dinner breaks.
An AGM battery with the same capacity as your pair of 6 volt battery will be a 4D or 8D group size, 2 to 3 times the size of a 6 volt GC2 group battery. You can buy 2 12 volt AGM batteries and wire them in parallel.
Most off the shelf 12 volt deep cycle batteries are hybrids that do starting and deep cycling, just not the best at each job.
A GC2 6 volt battery is built for electric golf carts, made to be discharged to just about dead by uncaring golfers and then recharged over and over again. Very popular and inexpensive because golf carts use 8 of them.
Don't mix up RC and AH, they are not the same.
The more RC or AH capacity, the longer it will power stuff before recharging.
Take all of the above into consideration before buying new batteries.