While this is a great theory, and I've certainly used it in the past, things have gotten a Lot More Complicated in trying to use this method...
You'll need to completely take one of the batteries out of the equation and work with the other one. You also have to realize that you have a myriad of electronics running when you turn on the key. All this stuff takes up to 1/2hr to shut-down so you'll be seeing the draw lessening as time goes on. Open a door, turn the key, mess with almost Anything and you start stuff back up.
It's a complicated thing to do now-a-days Plus, add to that, that the actual Amount of draw you're supposed to have (and you Always are going to have a draw) changes with how your truck is equipped..
Here's where I'd start, unplug the Alt from the truck. Pull the fuses for the Instrument Cluster & Radio. Then see if things improve, these have been the most likely culprits.
If it works, connect each thing One at a time and wait (for as long as it takes to show-up, overnight, a day, whatever you're seeing now) to find out where the problem lies.
If you get no change, thinks get harder
You can simply remove all the Even # fuses and look for a change. Then the Odd #, then 1/2 of the problem ones then the other 1/2 (you get it, hit or miss)
You can of course use an ammeter in series with one of the cables of a single battery and, once settled down, you should see under 2-3 Amps certainly. Pulling each fuse will show some have an effect but, again, some are supposed
to be drawing current at all times...
I guess it depends on how quickly you need this resolved
Also, you could look to see that the GPR isn't stuck ON. If it were though you'd be seeing a HUGE spark and it would kill the batteries pretty quickly (under an hr to both be completely dead) so I've assumed you don't have That going on...