There is a relay that is supposed to close when the ignition is on. It's the same battery cross-over relay that is operated by your Aux Start switch on the dash. Try that to see if it works. If so, the big relay is ok and the problem is in the wiring to the relay from the ignition. If not, it's often the relay itself. I don't know where it is physically located in your coach, but its usually very close to the batteries, to keep the big cables as short as possible.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL