Engine won't stay running with key in On position

[lumpy]

kinda dumbfounded here,  I removed the coach battery to recharge and only had the main engine battery in the Winne and took her into town for the last run before winterizing.  Drove fine  and stopped at church to do some handy work , came out and it wad dead.  had the wife bring me the second battery and just exchanged it and It would not stay running when the key was released to the run position.  well I have an old sunbeam and have been through that before and it is a sure sign of a bad ballast resistor. Now where the heck am I going to find one of those at 7:00 on a Sunday ? so out of a pure hunch I connected the second coach battery and the thing started up and ran perfectly fine?? i??   
Anyone  have any ideas or am I really just looking at a new ballast resistor.  My first thought is it is bad, but why would it run simply by hooking up the coach battery?

[OldEdBrady]

Bad battery?  Not enough juice to keep it running?  Several possibilities, and that's only two.

[DaveVA78Chieftain]

You jiggled some wires and now they are making a connection?

[Froggy1936]

Due to a wireing snafu the starter is operating off of 1 battery and the ignition curcuit is off the other battery  The starter curcuit also supplys the ign curcuit wile cranking with 12 V (reason it cranked and started but quit when key returned to run position) Then the run curcuit switches the ign syestem to 6V to run on  (reason for ballast resistor) Try disconnecting the 1st battery and see if it still cranks