So, my problem is thankfully not a parasitic draw but was a bad battery, which Costco swapped out.
This is *exactly* what happened to me!!!
Every time my 2005 Sienna sat around for a few days, it wouldn't start, and I'd need to jump it. I used a multimeter to test the battery (both off and running). That was my very basic mistake. This video here is what I watched: https://youtu.be/COJr7OB23Hw
So, thinking that the battery was totally fine, I then went on looking for a parasitic drain. The van would begin at about 1000 mA (1 amp) and then quickly drop and stay at 200-250 mA. I thought, "Oh, this is bad, because the recommendation is no more than 50 mA." So I discovered that after pulling the ECU-B fuse, it would drop to zero. Apparently, the ECU-B fuse is what is pulled when Toyotas are sitting in storage for long periods (like, before they go to the lot, etc). I drove the van again, and then it sat in the driveway for about 24 hours.
Then... IT WOULDN'T START AGAIN.
So I was scratching my head, thinking it could be starter solenoid contacts. My thinking was that the battery tried to send the voltage to the solenoid on the starter, but the contacts were too worn or dirty to send/receive enough voltage. So the battery needed to be at 100% in order to give enough power. If the battery was at anything less than that, then it's not enough to turn over the engine.
But I was wayyyyyyyyyy overthinking things. My dad insisted that I just take the battery out of the van and get them to do a load test on it (it's free). I insisted that the battery is fine since I checked it with a multimeter.
But in order to properly test batteries, they *need* to be tested with a LOAD TESTER. A load tester puts a load on the battery (separate from the car... you're disconnecting it from your car), and tests it that way. Otherwise, stuff like the alternator and whatnot is going to interfere with multimeter test, making up for what the battery is not able to do.
Well... the test at Canadian Tire showed that the CCA (cold cranks) on the battery were almost HALF what they were supposed to be. Guess what? The battery was also from 2008 (9 year old battery, lol). I replaced the battery, and popped back in that ECU-B fuse, and everything works as good as new. No more starting issues. And I still don't drive the van much (I'm at home with kids, so I sometimes I only use it once or twice a week).