In an already ventilated compartment, which is usually where the propane tank is or the battery bank.
However, the little 1# bottles do not have a vent and are rated by the NFPA as safe for indoor storage. That's why you can find them on store shelves, while the bigger (and vented) Blue Rhino tanks stay outside. I wouldn't lose sleep over it.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL