I'm noticing that the passenger floor underneath the mat gets damp. I'm almost positive it is related to the AC being on since there hasn't been any rain or moisture otherwise. I tried running some string trimmer line up the drain tube under the right passenger fender but didn't get any drainage or feel like I hit anything besides the bends in the tube. I've never seen any liquid come through there either and last summer I used the AC a lot (AZ).

Any ideas how best to troubleshoot this?

Do you see a wet spot under engine area after running AC during humid days? You should.
I think the AC evap coil drain tube spills out of the firewall on passenger side of engine compartment. Check if that tube is clogged.
Careful to not damage evap coil.
Or maybe dis-connected under dash?

No puddle, though humid days are hard to come by here. However, I feel like I should have seen something by now. I did check the tube with the string trimmer line but didn't unclog anything.

Anyone know or have pictures of the connection point? Can I get there without pulling the dash?

I know that you mentioned that the problem most likely isn't rain, but when I bought my truck, it had a similar problem. I found that the cause was that the cowl at the base of the windshield wasn't fully seated. On the underside of the cowl is a lip that diverts water away from the opening for the HVAC, and if it's not in the right place, water will spill down the windshield, through the cabin air filter, the fan, and onto the floor.

The windshield had been replaced at some point, and the installer did a crap job, not trimming off the excess adhesive which in turn kept the cowl from engaging fully.

Hope this helps. At the very least it's something to consider.
Good luck.

I'll try and look up what I used. But had a similar issue on the 04 V8 a few summers ago. After a long road trip found that the passenger floor was filled with water. Immediately panicked thinking it was heater core. No off color or smell and ultimately determined it was a clogged A/C condensation drain. There was an a/c condenser / coil cleaning kit that I got. It came with a long hose and spray can that you shove up the drain valve, and spray the cleaner up and it is supposed to clean off the coils and dissolve any blockages. I ran 1-2 cans of the stuff and followed directions on the packaging and my problem went away and hasn't returned since. Been a couple of years now.

Lubegard 96030 Kool-It Evaporator and Heater Foam Cleaner

You should be able to carefully pull plastic trim panel off the side and see the tube on the passenger side. You can drop the tray (there's one philips head screw that holds it on and then just pop it off, you can let it hang; no need to bother unplugging the modules) under the glovebox if you are having trouble getting it out/back in.

But that will let you know if it's connected to the HVAC box, you can pull back the carpet a little after taking the tray under the glovebox out and see if there's any holes or such on it's way out of the body.