Touring in Germany

Hi. We are visiting friends in Flachau, Austria, and it seems the best route is into Belgium then Germany and down via Cologne, Frankfurt, Nuremburg, Munich and Salzburg. We are going at the beginnning of August - I suspect it will be busy! - will we be able to turn up at campsites or stellenplatz without booking ahead? Don't really know how many miles it is realistic to cover in a day, so not sure where we would be stopping. Suggestions would be welcome! Being in a motorhome, preferably somewhere with easy access to food and drink  It will be a quickish down, but a more leisurely return.

Thanks in anticipation.