Mostly it's going to come down to how well the unit was maintained. An RV is more complicated than a car with all the "house" systems to consider as well as the engine. If you can find someone with a well maintained unit (who can provide records to that effect) you might be in business. Also, unlike a car, very low mileage might not be a good thing. If an RV is very low mileage for it's age, that might mean it sat around a lot and when an RV sits around, things start to freeze up or rot, seals get corroded, etc. And for sure you don't want a unit with any water damage or leaks -- a very common problem in RV's due to to bouncing around on the highway.
When it needs service you might have a problem finding someone to deal with it, unless you live in an area with a lot of RV dealers who offer service. Most of our regualr car mechanics won't touch it -- either their service bays aren't big enough or they don't know enough about it or they just don't want to deal with it. Also, most places that will deal with the engine won't deal with the house systems and vice versa. We have a whole team of people who do various work on the RV -- one for the engine, one for the house systems, and another guy for the tires and another guy for the state inspection each year. This is one thing to find out about before you buy -- you don't want to need service and then have to start digging around to find somebody. If you're mechanically inclined a lot can be fixed yourself, but you need to know a little about engines, HVAC, plumbing, electrical and mechanical to do much service yourself.
And when you're figuring your costs, don't forget to factor in insurance, title/registration fees, property taxes and maintenance. And then you have to outfit the thing with linens, kitchen supplies, and anything else you need for a trip (it's a litttle like going to college and outfitting a dorm room). It may be more expensive than you think.
There's a lot that goes into an RV and everybody's needs/wants will be different, but I would suggest starting out by going to a dealer and looking around at the new ones -- talk to the salesman and get an idea of how the systems work, what looks "normal" etc. Armed with that information you'll know more about what you're looking at when you start looking at used ones.