I'm not a gear head but personally, and generally, I'd tend to lean toward the 8 year old B with the higher miles. The mileage indicates use and suggests to me that there is less of a chance of gunked up lubricants, degraded belts, frozen mechanical parts, worn / outdated appliances and electrical / plumbing / systems, etc.
I'd prefer the newer appliances and possibly other newer technology in the 8 yo. Parts would also be easier to get for the 8 yo vs. the 12 year old.
By the same token, the 12 yo could have been taken care of with the fervor of an obsessive-compulsive, whereas the 8 yo could have been neglected.
There's also the issue of resale. All things being equal, I'd guess the newer unit would sell more easily.
I'd have a trusted / honest mechanic who is also familiar with RVs check it out.
Good luck!