Hi Geoff

I had exactly the same thing happen to me with my first camper van. The dealer tried everything but never got the system clean.
In the end he replaced all the piping, the pump and the water tank. The water heater was cleanable because it was stainless steel inside. The water tank he put in didn't fit properly and eventually he had a bespoke exact copy of the original made at great expense.

Whatever you do don't try to clean it up just take the whole van back and tell him to a)sort it and b)report back to you exactly what he has done to put it right. Be persistent or he will try to fob you off (as mine did with the first water tank) as it will cost him big time in parts and labour.

His insurance should cover it - if he has any and you should have got a short warranty with the van as you got it from a dealer.

Good luck and if you need more info you can phone anytime I will PM you the number. Mary