We drove there from somewhere else, but with your campervan I guess you will want somewhere nearby. For motorhomes Camperplaats de Haamstede is close to Giethoorn village and gets recommendations but also warnings that it is very busy and does not take reservations. The village is a tourist hotspot and there are about ten campsites in the area as well as that Motorhome site.
I am planning a Dutch trip at the moment, going in a couple of weeks and Giethoorn is one of the places I have on my list to visit.
I am intending to stay at Camping `t Kappie at Steenwijk which is about 7 miles away but looks to be an easy bike ride in, as is just about anywhere in the Netherlands.
If you look at the ACSI website it is no. 265 at 14 euros a night out of high season. It also gets good reviews on several sites.
I read a very interesting article in The Guardian on-line yesterday about how the Netherlands government are worried about too much tourism ruining parts of the country. One of the places discussed was Giethoorn which, with a population of 2500, gets 350,000 tourists a year from China alone.
Don't forget to leave a review of the French and other European campsites you have visited!