Extreme Parasitic draw

Here is a video on how to test for a parasitic draw... https://www.youtube.com/watch?v=QRso1A0VScw

I would hook it up to a steady 12V source, and also make sure to pull driver door fuse or computer modules will stay "awake" and draw more current than when they sleep, usually about 10 minutes after closing the door. That's a huge draw from your description, sounds like a motor or one of your add-ons.

I would start by disconnecting the home made add ons and see if the draw disappears.

First, I would disconnect the battery and recharge it. Then test to see if it dies by itself from an internal short. They do that when they get old. Next, I would disconnect the alternator output wire, the heavy red one and hook the battery back up and see if that was the problem. Sometimes alternators develop a "leaky" diode that bleeds the battery down even when everything is "off". Next, I would start testing each of those add on gadgets for drain. Until you get it fixed, park it outside or disconnect the battery so you don't have a fire. A dead battery in ten minutes is a huge drain! I hope this helps.

Thanks so much for the advice! I'm headed out today to try it out... I charged both batteries disconnected this time and I'll try the test light out. So the previous owner was the one who put in the add ons, and I think I know which one is the problem. There is some kind of coolant heater that he installed inside the cab, behind the back seat. I'm not sure where the power originates but I think it's direct to battery. I didn't ever hear the motor when the batteries were draining, but could that mean the motor was just seazed? Also if I disconnect it, I should be able to cap the coolant hoses without affecting the system right?

Also he has a 4 plug, in home style outlet wired with the tail under the hood for plugging into a wall outlet, with THREE cords plugged in that look as though they all go to the block heater. Is this set up for the block heater normal? Or is that also likely to be causing the problem?

Ok, so I messed around with it for a while and figured out that it's actually something to do with the starter solenoid. More precisely one of the circuits connected on the same post as the wire from the battery to the solenoid. I removed that wire and tested it with my meter, it has a short to ground somewhere in that line... I don't know what that circuit controls though but it's a larger wire that splits into 4 and then follows a bus to all the connectors under the steering wheel. I'm not sure how to figure out which circuit it is... Do I have to brake open the wire bus to follow those wires? Or is there somewhere I could get cheap wire diagrams? I also tested the starter solenoid and that works, also tried the screwdriver start and the starter cranked just fine! Is just when that wire is connected, it somehow draws all the power that should be for the starter!