There are camping and motorhome sites all over NL, very good ones indeed (the Dutch virtually live in caravans on holiday, and to my fellow residents in NL who are reading this this is a compliment, not sarcasm!!). In Amersfoort there are three very good ones within about 4 km of the event site: there are others within a 10 km radius. There are in each major town usually a number of very good campsites: if you want to bring the motorhome, then I doubt there will be any problems with you parking at the event itself - there's a HUGE car park, chosen specifically for that reason....