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I installed a kill switch because I found that something is SLOWLY sucking my batteries dry. I assume it is the starter switch, since that is the only thing that is always "hot" with mil-spec wiring.
(1) Is this a common issue & is this diagnosis way off base?
(2) Would a simple cleaning/rebuild of the starter switch cure this?
The kill switch solves the problem, and I like having the "oh, s**t" option of disabling most of the electrical system on demand. It is just those few times I forget to kill the electrical system to return to dead batteries.

The starter switch could be grounding out internally, but that would normally result in fairly rapid draining of the batteries. Bad insulation on one of the battery cables where they come through the floor is another possible cause, but it would also be a fairly rapid drain. A slow drain indicates a continuous current flow through a relatively low amperage device, such as a brake light that doesn't turn off when you take your foot off the brake (hung up switch, usually). It could be any number of other things, too. Happy hunting! It'll probably be a bear to track down, but dead easy to fix once you find it.

Disconnect your battery ground cable. Disconnect both the positive cable and wire #4 from the starter switch. Connect a test lamp between the ground cable and the battery negative post. If the lamp glows there's a short between a battery cable and ground. If the lamp does not glow the battery cables are ok. Remove the lamp. Re-connect the positive battery cable only to the starter switch. Re-connect the test lamp to the ground cable and negative battery post. If the light glows the switch is internally shorted.

Due to high temps and arcing and moisture invasion the buildups inside the switch will eventually develop into a ground path much like carbon tracking in a distributor cap and initially the battery drain will take several weeks but eventually it will become quicker.

If the light did not glow on the second test disconnect the test lamp. Now re-connect the #4 wire with the battery positive cable at the starter switch. Re-connect the test lamp at the grounds. If the lamp glows you have a leak downstream on #4 or further into the regulator.

To eliminate the regulator just unplug the #4 wire's cannon type plug from the regulator. The test lamp should go out. If it does not then you have a short somewhere in #4 wire. If the lamp goes out then connect a jumper wire between pins A and C in the disconnected plug. This will apply voltage to the #10 wire and on thru the circuit breakers and main switch. If the lamp goes out the regulator is at fault. If the lamp glows then your regulator is good and the problem is under the dash. From this point on using your appropriate 37 or 37B1 wiring diagram you simply disconnect one circuit at a time from the #19 feed in until the lamp goes out.

Jim, it's never a good idea to leave something like this unchecked. I'd search it until I could locate and correct the issue at fault. You have been given a good guideline to search. You just can never tell how serious such issues may actually be, I know you have the master disconnect, and that's good, however a burn down can happen while driving down the road. Just not worth taking chances, everything can go up in smoke really fast.

Well, the igniton switch checks out ok. On to the starter ...

I was just installing a trickle charger on my truck. The negative disconnect was off and when I tightened the negative nut on the battery, I got a load spark when my wrench touched the battery box. I am a little concerned now and I think I will try to trouble shoot whatever is trying to drain my battery.