Most modern RVs will be fairly reliable as vehicles; the drivetrains and engines and such are pretty good. There is some variation, of course, but the major chassis are all pretty well proven and last well given appropriate maintenance.
The coach/house part is somewhat of a different matter. Don't expect Toyota levels of reliability there from any manufacturer, though some come a lot closer than others. The appliances and other mechanical systems generally all come from the same few companies, not the coach builders, so there's not too much to differentiate there. The quality of the wall and roof construction and care in sealing and that sort of thing do vary a little more, and all require more or less maintenance and vigilance to keep in good shape.
Partly, too, the systems are a lot more complex than in a car, with more things to go wrong. A seat and a strap to hold you into it is pretty trouble-free; a refrigerator, plumbing system, stove, and furnace are rather more complicated and fiddly.
Initial quality is not the strong point of most RV manufacturers. Usually things improve a bit once the initial set of manufacturing faux-pas are corrected (generally under warranty). A lightly used unit might be a better option for that reason; someone else has gone through the fun of getting fixed things that should have been fixed before. Some of the smaller more boutique makers, like Phoenix Cruiser or Lazy Daze for example, generally do better in this regard (but are also less affordable).
