They have been known to fail.
"Use/store switch bad" search results:
https://www.google.com/search?q=use%...=airforums.com
The light on ours does not always illuminate, so we turn it on and off a number of times in quick succession, after which the light usually comes back on. Maybe try this a dozen or so times?
When you say "All systems otherwise seem to work" do you mean all the 12-volt systems function correctly when you disconnect from shore power? If you have an overhead light on for instance, and then put the Use/Store switch on Store, does that overhead light go off? Or does it stay on no matter what you do with the Use/Store switch?
Good luck!
Peter