I an beginning to agree with Webasto re High Sulphur content fuel, based on my experience.
My father in law was a Logistics Manager for one of the countries bigger Haulier's with almost 300 vehicles under his control including the maintenance budget. I asked him were the 'Night Heaters' (as they are known on Lorry's) as much of a problem as on boats, his answer was they virtually never had a problem with them.
Seems to me that fuel is the key culprit and it does make one wonder about fitting a separate fuel tank and run your Webo with White diesel. Trouble is I might not be able to claim the 40% rebate for my heating