I camped the UK summer 2002 and France this summer. We found it was cheaper to buy a used motorhome and tour with it rather than a Eurail pass. We bought a used Mercedes motorhome, used it for two years and sold it for more than we paid for it.
But, transportation aside, the camping in Europe is great. There are many campsites in or near to cities and the facilities are usually excellent. There is a campsite in Versaille that is excellent and just a few hundred yards from the train station that only takes 20minutes in to the center of Paris. You'll see lots of tenters and a huge array of different kinds of tents, much more variety than in the US. Given the fact that there is generally more rain in Europe, I'd look for a tent that I could take down while the fly was still up so the main body of the tent would remain dry.

The Lonely Planet guides are good for campers. They are aimed at backpackers and pick he sites readily accessible from public transportation.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13