I lived in Europe for over 10 years off and on thanks to Uncle Sam, and go back periodically both for work and fun. Some of my adventures include the rail pass thing for 4 weeks using a backpack, and either campgrounds or hostels as appropriate, also did a fair bit of touring by bicycle, the longest being John O'Groats to lands end in England. One thing worth mentioning is many of the campgrounds are more Holiday camp, which I loved, but you may find loud and crazy if you're more used to American style campgrounds.
For the most part you aren't going to want to drive an RV in any Major city. Most have congestion zones now where you pay extra to drive each day, and you will spend much of your time in crazy traffic. For the most part you'll not be able to camp with an RV in the cities anyway, so use mass transit for your site seeing of the cities, which you'll want to do a lot of. If you're at all into history you'll have the time of your life. Keep the RV you end up with as absolutely small as possible. You'll really appreciate the smaller size if you venture off the main roads at all. Unfortunately, the countryside is hard to explore without a car, and it's well worth exploring so you'll want something you can explore with size wise. Finally, as has been mentioned, fuel, along with just about everything else tends to be quite expensive in most of Europe, even when the exchange rate is good. Plan for it so you don't get caught coming up short.
Good luck, sounds like a trip where lots of memories will be built.
