1) Ditch the cover. Does more harm than good,
2) Assuming your batteries are FLA, just (a) make sure they are fully-charged and (b) disconnect the grounds. No need to take them inside or to trickle-charge, and the latter carries some risks.
3) Be happy that you will be unable to start the engine. It is far, far better to just let it sleep. I can't speak to the Onan, though.
4) Leave a few windows open just a crack. A little circulation goes a long way.
5) Relax and enjoy your trip. It will be fine.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)
Next: 2022 Fully-custom buildout (Ford Transit EcoBoost AWD)