On the ac question, we have a single 15k in our similar Eagle. We also camp in the high desert in summer with temps near 100. Sure wish we had a 2nd unit in the bedroom. You still have a choice. And if the Whisper Quiet is great, fine. Jayco says it's available in 15k.
As for whether the salesman was being truthful, were his lips moving?
The price does seem fair.
'11 Eagle 320RLDS
'02 F350 PSD Dually 4WD
DW's Ride, '13 Expedition