I personally would save teh campsite fees and sleep at motorway service stations (I do this fairly regularly in germany, and there are always loads of campervans). Also, many German towns have free campervan parking places, look for the signs on teh outskirts. Also they have places wehre you can empty the sewage tanks if you need to.
The loos in aires (motorway lay bys) in France and Germany are free, but the ones in service stations arent.
You can fill water bottles at local fountains in France (but check for the sign saying "Eau non potable." It may have been put up by the cafe selling expensive bottled water next door. Then again, it might not.)
For showers, you can often dunk your children in the same fountains, or use beach showers.
tip: dont drink or dunk if the water coming out of the fountain is warm on a summers day. That means it is the same water circulating, instead of coming out of the ground.
Petrol is cheaper in Luxembourg than in France or Germany, and its cheaper in France than in Germany.
Hotels are cheaper in France than in Germany.
Sleeping out in the Alps is jolly cold, even in summer.
Try to be in Koln in the first week in Feb for the Karneval, and Munich in the last 2 weeks in Sept, for the Oktoberfest (yes, I know).