Hello and welcome to the Forum.
I have a 2006 Fleetwood Tioga that has the same battery box area as yours,
When I had the same problem as yours that the switch would not work and when the engine was running it was not charging the house batteries,after some testing I noticed that the
continuous duty solenoid would click but not lock in which is a loud clunk. It is where those 2 big cables attach at the lower right of the picture.
After replacing it the switch
worked and the engine,thru the alternator,was charging the coach batteries.
As for the purple wire, on mine that black box at the top of your picture is a BIRD.On mine a purple wire is connected to the Ignition terminal and one to the Relay terminal.
I hope this helps,any more questions come on back and ask or PM me.Let us know if this works.
