I would keep the ammeter connected and start pulling fuses. Take a photo of each fuse box before starting and make a list/diagram of what amp fuse goes in what slot. Don't put them back in. Pull a fuse then check the ammeter. No effect? Go to the next fuse. I would think that eventually you're going to pull a fuse and that draw will stop and you'll know what circuit has the problem.

That's how I would approach it. You have a real head-scratcher. I know there is a draw on vehicles now for a number of components but that seems to be very high. We left our Sienna in the garage for 2 months and when we came back the battery was stone dead. Now when we travel I connect a charger to the battery and put the charger on a timer; half hour in the morning and again in the afternoon.