Dual Battery Size
That sounds about right. I vote for the two 6-volts. I had the same issue with the 233S and, unless I wanted to modify the tray, the deep cycle 12-volts that would have provided me the same Ah in 6-volts were too long.
So, if you dry camp often, buy the most Ah you can afford and fit on the tongue and generally I found that to be 6-volts in series. Take a look at Trojan T-145's at 260Ah at the 20-hr rate.
I don't carry a spare 12 volt in case of emergencies. Its rare to have a sudden battery failure, I think, and you should know when one is going bad with periodic testing of each cell's specific gravity. I would just go buy one if needed on a trip. But, if you are going to carry a spare, than think about a sealed battery such as an AGM rather than Flooded. The AGM won't leak if tipped over, which can happen if carried loosely in the bed of your truck or in the camper itself.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD