Delivery trucks, shuttle busses, and other commercial vehicles built on the same chassis as motorhomes often go for hundreds of thousands of miles. Mileage (or lack thereof) in itself would not be a primary concern for me. I do think that, all other things being equal, lower mileage means less wear and tear on the drivetrain etc. and would be slightly preferred, but should not be the biggest concern. There are some chassis parts that tend to need repair or replacement due to age, more or less regardless of mileage; there's a good chance that fifteen year old brake calipers may need rebuilding or replacement, for instance. Some seals and belts and hoses and bushings and such also deteriorate with age.
I would avoid units that are old enough to not have fuel injected engines.
Like wrvpo said, check the house and house systems carefully. That's where you'll more likely have work to do and more maintenance to keep up with. A 2012 unit with 10K miles could be in great shape, or could be subject to lots of deferred maintenance. A 2012 with 60K miles could be very well maintained or beat up. (At least with 60K miles the tires would hopefully have been replaced once and have at least a couple more years left in them.)