Thank you, Chris - of course I have electric valves on the furnace and fridge. A senior moment for sure! Yes, I see my idea that a regulator might deliberately have a very slow leak is also wrong - over pressure protection requires a valve and large vent. Thanks for the tip to replace my 11 year old regulator. And hoses, I reckon.
It must be just little leaks in various places, with the electrically operated valves high on the list. The time it takes for the air in the lines problem to occur might be a useful indication of how leaky the system is.
I see an earlier discussion of this here:
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/22835405/print/true.cfm