RJL,
While I cannot supply a specific answer for your coach, I can tell you some things that are very generally true.
Most coaches will charge the house bank from either the APU (genset) or shore power.
If the house bank is anything like down, an half an hour will do no good at all.
An half an hour of running the APU that is only charging the house bank will do it more harm than good. If you feel you have to run it, then run it at a least one half load and for an hour. You much better off figuring out how to drain the fuel out of the carburetor an then leaving it alone.
Nothing good happens to an engine that is idling.
Matt