I am having the EXACT same problem. If the Atwood fridge is hooked up to electric for more than a day and then I unplug, it will not auto switch to gas. Eventually goes into fault mode. Any type of manual intervention is unacceptable to me.
My trailer is only about 4 months old and it is in the shop now, having had a brand new (2nd) fridge installed. New fridge is having the same issue.
Dealer has tried everything and is now doing more testing as recommended by Atwood this morning. After making the new recommended tests, the dealer is to plug in the fridge for 24 hours and see if it goes to gas when unplugged after the 24 hour period.
I'm definitely thinking Atwood has an issue with this fridge.