A modern motorhome drivetrain, with decent routine maintenance, will easily go for a couple hundred thousand miles without needing replacement of major parts. The engines and transmissions are the same as are used in many many vans, delivery trucks, etc. Of course, without maintenance, they don't last as well.
Motorhome structures do leak, and constant (or at least periodic) maintenance is necessary to prevent it. I'm sure I'm not the only one who would like to see the house structures built using better techniques; car bodies have managed to stay more or less leak-free for decades without continuous sealing and maintenance, and there's no technical reason why a motorhome couldn't be built using similar techniques and have similar longevity—indeed, the Blue Bird Wanderlodges come to mind as an example along these lines. It would be more costly (as they were). Proper fiberglass construction can also be long-lasting and largely leak free with minimal maintenance; some small travel trailers in particular are like this and work quite nicely. I think the GMC motorhomes also were of fiberglass construction (rather than just fiberglass sheeting applied to plywood).
Finding a 20 year old motorhome which was stored outside and has never had any leaks whatsoever is probably not too likely. Finding one where any leaks were properly and promptly repaired, and where there is no significant structural damage, is more readily possible, and such a unit can give many more years of enjoyment with good maintenance. It is, of course, very easy to find old RVs with significant water damage.
I've heard it said that the difference between an RV and a boat is that in the RV the water can also leak out!
