On my '75 Monaco there is a "Field Load Relay" on the firewall that only allows current to pass from the alternator to the battery (and other heavy current draw parts) when the key is on. It's a small metal cube-shaped box just to the passenger's side of the wiper motor. When you turn the key to ignition "on" (not "start") you should hear it click. The way that I found out that mine wasn't working (other than it killing the battery) was that the heater blower motor also wouldn't come on when the key was in the ON position. But, this relay is not available through normal auto parts stores or from the dealer. I may have to figure out how to make/create/rig a different relay to take its place if/when this one dies.
"Boys, you oughta learn not to talk to nuns that way!!!" - Curtis
Our Bluesmobile still lives on YouTube! Look for "Shotgun Blues Brotherhood" and pick the thumbnail with me and "Jake" sitting on the hood of the car.