Well, Old Biscuit, normally I'd agree with anything you say, but in this case, I must respectively differ. I checked the by-pass valve. There is a diagram right above it, indicating the handle position for "winterise" and "normal". Can't be but one of two ways.
The shower has two separate knobs for cold and hot. The problem shows up when only opening the hot faucet. Both outside shower knobs are closed. In fact, I have lost the key for the door, outside!
What we gonna do, in a few minutes when the DW steps in the shower and gets three seconds of hot water? And the propane switch is on, but not electric? Or even worse, it's normal on propane and acts up only with electric?
Ok, here's the report from the Mrs., same problem with propane as heat source!
Someone else care to ring in? Old Biscuit, any other thought? I agree with you, by the way. I just can't figure out how it's happening. The diagram I saw was on an Atwood site. I don't really know how I'm gonna check the integrity of the cold water inlet.