Mid-level comfort

Self-Drive motoring holidays in Europe (http://www.drive-alive.co.uk/) for the independent motorist. These are camping and mobile homes in France, Italy, Spain, Switzerland, the Netherlands and Luxembourg. The minimum stay is three nights. The cost of your camping is included in the package price.
To check out prices I invented a family, parents and two kids, with a saloon car coming from Britain in May and spending five nights camping.
The answer came back: The price of your holiday is €404 ($US508) including accommodation and crossing, but excluding insurance and maps.

Bring your own tent in true camping style

Interhike, http://www.interhike.com, covers UK, Ireland, Spain including the Canary Islands, Germany, France, Portugal, Czech Republic, Denmark, Poland, Sweden, Norway, Finland, Switzerland, Italy, Greece, Belgium, Holland, Luxembourg which is pretty well most of Europe.

Note that this is both a listing of camping sites and a booking centre. It does not guarantee the quality of the sites although, from experience, outside of the annual European holiday in August they are generally quite splendid and civilised. In almost every case you have to take your own tent and gear.

This is just a brief sampling. You can explore the Internet until your eyes grow dim. Use a Google search and just type in camping Europe. Then pick and chose.