In my opinion, there's a bit of research that ought to be done beforehand. Additionally, what kind of weight is the vintage camper?
First, what engine/transmission/rear axle combination? Based on your objectives of towing a camper, this could very well make or break any deal.
Second, does the owner have records that establish a plausible mileage? Can you post any photos?
Third, have you looked on ebay, craigslist (multiple listings throughout several cities), autotrader (and vintage auto trader), swap meets, etc. to compare/establish market value for that particular truck with its' options?
To add air conditioning can cost up to several thousand dollars; depending upon new/used (or refurbished used) parts, installation of factory style or aftermarket style, etc. Several of the add on aftermarket type A/C units do not keep the interior in a vintage/factory look.
All in all, if the truck is good for you, then it ultimately depends upon if $12,000 is a good price to you...