I drove some 400 miles today and much to my surprise, the hot water tank, for some reason, didn't get heated as usual. Usually, after a days drive, the hot water would be very hot.... enough so to last a couple of days without needing the electric heater element turned on.

A couple of months ago, the engine coolant was drained and replaced, and an effort was made to isolate the webasto plumbing circuit at that time, so I'm guessing that enters into the mix.

So, what am I missing here? What should I be looking for as far as the valves are concerned?