This is one of those dumb things I should have known about, but had no clue. Late last fall, the fresh water pump started cycling everey two or three minutes. I couldn't figure it out, and it was the end of the season, so I just turned the pump off after using the sink.
This spring, the SMB started to get a slow fresh water leak from the area under the sink but below the living area on the driver's side here, right at the city water connection.
Taking a look underneath the connection, it appears to be coming from the pressure reducing valve here:
Sure enough, this hairline crack on the pressure valve housing was causing the leak and the pump to cycle every few minutes.
The dumb part about this, is that I never realized that you can drain the freshwater system from the small valve beneath the pressure reducing valve here.
I have always drained the fresh water tank from the valve under the bench seat in the EB-50, which does not apparently drain all the water from the lowest points in the fresh water system. I almost never use the city water connection, so I hadn't seen that valve beneath it. Low and behold, that valve is described in the Sportsmobile Owners Manual, but I had not studied the plumbing diagram close enough in previous years.
It looks like the pressure relief valve froze in a sudden frost early last September, which caused the crack.
The question is, has anyone else had this problem, and had to replace the ShurPro pressure reducing valve? I can't believe I didn't notice this drain valve before and that it took three Montana winters before it froze and cracked from a little residual water in the system.