Condition is everything, I would much rather buy a 10-15 year old well maintained coach that has been stored indoors, than a 3-5 year old coach which has been neglected. Mileage is rarely an issue with most privately owned motorhomes, generally speaking they tend to accumulate only about 6,000 miles per year on average, and are built on commercial truck / van platforms with a 150,000 - 200,000 mile designed service life. Therefore a typical 10 year old motorhome will have only around 60,000 miles on it.
On the topic of condition, rot of one type are another is the prime killer of RV's mostly water intrusion from poorly sealed roof penetrations, roof damage or window or slide seals. This water damage is likely far more extensive, and far more expensive to repair than it may appear at first glance. In the overall category or rot, you have to also worry about rubber parts dry rotting on older coaches, tires should be replaced about every 7 years regardless of wear, other rubber parts, belts, hoses, and rubber bushings tend to last only 10-12 years before becoming brittle and starting to crumble. There are a lot of such parts, way beyond just the main serpentine belt and radiator hose, included in this list is rubber brake hoses, the generator fuel line, the propane regulator, the various suspension bushings, ball joints, steering linkages, sway bar bushings, bump stops, etc. I just had one crumble in my hand while changing the starter on my 18 year old Onan generator last week (the crank case breather tube, which delayed the reinstalling the generator by 5 days while waiting on a part).
As to craigslist, many of those apparent deals are really money pits waiting for the uninformed first time buyer to come along, many others are scams, this is not to say all are, I bought my current coach off a craigslist add, just be aware it is a buyer be ware environment and coaches that look good in selected low res photos, may look completely different to an experienced eye in person.