We fitted an Eberspatcher D4 in 2002 but as we have just had to replace the clever bit of electronics that controls the heater, which was not cheap, I cannot say it has been completely trouble free. But it has functioned well for 9 years without ever being serviced. When I stripped it down it was relatively clean and there is very little that appears necessary other than replacement of the metal bit that fits into the heater plug hole, which you buy as a service kit.
We took the unit off the boat in Brittany and visited our local Eberspatcher dealer in Preston who checked and found what was wrong and provided the new control unit. The service was good with no charge other than for the parts.