I also suspect you might have two 12-volt and from what I hear, the OEM batteries in the Thor products around that time were pretty bad (Harris batteries?).
You might take a look at THIS POST
as this person is posting about a 2015 Challenger and was able to fit 4 -AGM 6-volt batteries in the tray with reportedly no problem. AGM = maintenance free. He even gives what brand he purchased and the specifications plus installation tips and a photo.