I think that's a bit high myself. We just traded a 2003 Kiwi 17a that was in immaculate condition, no leaks, no smell, etc. The dealer was asking less than $5k and it sold in less than a month. Check the floor for soft spots and the sides for signs of delamination.
In theory the Flex should be fine, but you may not like towing through mountains or hilly terrain. I'd also find out if there is a frontal area limitation to the Flex. Towing a box, even a small one will put a strain on your vehicle.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE