Wow, so many options puts you in the drivers seat!! Although the 2012/2013 have barely hit the road they may need some maintenance i.e. the roof resealed, bearrings repacked, annual check for gas line leaks, annual window seal check, check tire date (not tire wear) etc. So, I would ask the owners for maintenance records.
I once called on a trailer and the guy had the trailer for 4 years and didn't know how many miles he put on the trailer and never repacked bearings!!! It's my personal opinion that you will have less headaches down the road if someone ATLEAST did the annual maintenance on the rig. If the owner provides the records they more than likely valued the AIRSTREAM brand and took really good care of their rig.
If it were me, I'd chose between the 2013 and 2014. Ask for maintenance records on the 2013. If they don't have any Id stick to the 2014, less foot traffic than the 2012. If the dealer won't budge on the price of the rig ask him to install items such as solar and inverter at no additional cost.
Just as a side note my friends just sold their 2010 for 28k. They had multiple offers and it sold in 2 days.
Best of luck!