Though it could be your key switch, Richard, there’s a greater likelihood it’s the solenoid the guys are referring to in the electrical bay, street side forwardmost. See Mike’s diagram.
If there is power on the right side large post of the solenoid when the key is off, and the small left (+) post is showing no voltage with the key off, then you need to replace that solenoid. It is a common failure item reported here. You need a “continuous duty” one rated 85 amps. Most here seem to be using the Cole-Hersee 24059. There is a less problematic solid state electronic version, but it is very expensive and most opt to just carry a spare standard one since they’re relatively easy to swap out. Just keep the diode (between the small posts) positioned the same on the new solenoid as it was on the old, with the (gray?) stripe to your left (+ side).
I’d turn off my Main 12 volt Chassis switch in the battery bay before changing the solenoid, just to be sure you don’t have any arcing or have a mishap with a wrench in there.
Reference this prior discussion:
http://beaveramb.org/forum/index.php/topic,8088.msg56152/topicseen.html#msg56152Joel