Given the age of these two motorhomes, condition of the coach part IMO is more important than mechanical condition. I'd pick the one that was the cleanest, had the most complete service records, and gave me the impression of being the best cared for. I would certainly invest in professional inspections to help you make a decision. All things being equal, I'd pick the Ford chassis for the aforementioned parts availability. Nothing wrong with the GM chassis either, both have roughly the same performance and MPG. The Ford will rev a bit higher to do the same work.