I'm not sure what I'd recommend on an older motorhome, so much depends on the individual example itself. I would be inclined to look at one's that were maybe late 1990s, as you would
a) probably find any spares for the vehicle were easier to obtain - once you get to 20 years old spares can be difficult - the only exception being perhaps with Mercedes, of which there are so many of their vans worlswide that spares are still reasonably readily obtainable. The other 20+ year old base vehicle I might look for would be a VW, but there are not too many about
b) you are more likely to find a diesel - which are generally more economical
c) everyone seems to swear by Hymer - which would be available across a wide range of ages, and can be found on Mercedes base vehicles
If you go to about 2000, you would get a common-rail diesel engine, which are generally more economical and quieter than the older diesel engines
[As I have written this, I have just recalled that an acquaintance has a Hymer, on a Fiat base, I believe, a few years old which he uses very little and says he is thinking about selling. Mind you, he has said this before and not sold it. I'm pretty sure it is left-hand drive, which would be no harm if you're going to tour Europe as well as UK. I don't know a lot about it, but will make some enquiries if you wish? When are you planning to come over for your trip?]