Ok, this isn't a wet blanket post. I'm just throwing out another viewpoint for consideration.
I don't know what you'll drive to Mississippi from NC and back but it seems to me you'll have a fair amount of time and money into the trip; all to (maybe) purchase A/C units in unkown condition.
Brand new RV A/C units are not extremely expensive and come with warranties. Personally I'd rather skimp somewhere else, save the cost of the trip (and the possibility the units are not worth it), save the time, and find good pricing on a new unit (eBay maybe?). I would also think there must be some take-outs or refurbished units around in the Southeast as well.
As I said, this is just food for thought.