No reason to go with AGM, unless you are mounting the batteries inside the camper. They are much more expensive without any additional capacity for the equivalent size. AGMs do have an advantage of a lower self-discharge rate, but as long as you can charge your battery once a month (the minimum necessary for a flooded lead-acid), the extra cost doesn't give you any advantage there, either.
Whether you go with two 12 volt or two 6 volt batteries, its best to use them together to take advantage of the "Peukert effect", which basically gives you a few bonus amps. And it is best to use two 6 volt batteries in series since it avoids some issues you get with paralleled batts.