The Harwich - Hook crossing is long but at least you get two free meals in each direction which is a big cost saving. We have been to Koningshof and enjoyed the site and friendliness there, but the pitch was very small - cars parked seperately - we had to push the van into place as there was not room to do so with the car and the neighbour had to take down his awning sun canopy to let us in! Then a motor caravan and tent pitched next door and had to circumnavigate out hitch head to walk round his unit. Not all the pitches are this small but you all pay the same! Don't forget to take your bikes as it's VERY cycle friendly, but we found the roads congested an the signs difficult to read!
Calais - Dutch Border is a 3 hour drive (try a stopover in Gent, beautifull small city) and will save you a lot of money. The Randstad area Rotterdam-Amsterdam-Den Haag-Utrecht is very crowdy. Better places are the regions of Zeeland for the seaside, Drenthe for woods and Limburg for hills. Look at http://camping-nederland.pagina.nl/
for campinggrounds or www.holland.com for general info. Good luck from Holland!
We have been the last 3 years and stay in Schoneveld near Breskens in Zeeland. It is right on the beach, with lots to do. The plots are huge and the people are super friendly. It is just a 19 mile trip from Zeebruge, and we have always used P&O from Hull and treated the 2 overnight crossings as part of the holiday. We live in the north of england, so taking into account the fuel time and driving, we have always found the cost favourable.
The have a website, just type Schoneveld on google