i have a 92 f150 5.8L, had alternator issues and starter issues in the winter and replaced both. shortly after replacing them the battery would drain in a couple hours of the engine being shut off. checked all the fuses none blown including the fusible links. tested all the wires in the charging and starting circuit for continuity and they are all good. the one way i found to fix the problem is really messed. from the ignition switch runs a green wire to the alternator. i had to attach a wire onto this green wire from another constant power source (tapped into the engine bay light wiring) with a switch so it doesnt continue to drain when the truck is off. if i start the truck with the switch i wired in off it will die shortly after. if i start it with the switch on it will run perfectly fine. the weirdest part is though if i dont have the switch hooked up the battery dies. however if it is hooked up and turned off the battery is fine, though if the switch is left on the battery will die as well. if anyone has any idea what it could be that would be awesome.

 

Sometimes alternators short out internally and cause a draw on the battery.

 

The Zener diode idea is a good one.

Could you just only allow power to your DC-DC converter to only be active when there is a "Run" wire that is hot AND the upfitter switch is on? Sort of a feed from a 12v source that is only active when in 'run' that would activate a relay that would then allow the feed from an upfitter switch to activate your large relay powering the converter. That would include at least 2 relays to control the relay you use to feed the converter.

I've got a crazy circuit I built to activate my aux tank fuel pump that requires an upfitter switch to be on AND I have to press a momentary switch to activate a timer so only 10 gallons pumps and them shuts off. It can only be activated again if I press the momentary switch (which is also lighted when the circuit is active). That way when I forget to turn the upfitter switch off the pump doesn't accidentally run when I start/stop the engine or put the truck in ACC mode.