As far as i am aware, you can buy a Webasto and a box made by them for it to sit in. I am sure this is what a dealer in the U.K said.
I have had a hot system since i started out waterfed pole. In the winter it keeps the pole and hoses warm and supple and the windows dry up slightly quicker. In the summer it removes baked on bird poo alot easier. First cleans are a bit quicker, and i find that on ali frames that oxidize, if you scrub and rinse well on the first clean, then use hot thereafter you dont have the problems of runs and spots on the subsequent cleans, imo.
The cons, well it costs about £5 a day to run, i dont notice this and i dont know if thats true, as my system is plugged into the vans diesel system so have no idea really the true cost. Would i go back to cold, in a word no!
www.webasto.com/home/en/homepage.html if you put in Netherlands it will give you a dealer near by to you. You could have a word and see if they would fit one for you to your system, im sure you could find somebody that would.