Engine repair question about 2008 Town and Country 3.8L

I haven't played with the 3.8 at all, but you might check the spark plugs to see if they confirm coolant is going through the cylinder. Typically it will wash the plug because of all the steam, sometimes the porceline will turn colors.
I'd stop at an auto store and rent (typically free) a pressure tester to see if you can pressurize the system and see if that helps track it down.
Again, not familiar with the 3.8 but a leaky intake sounds like it could result in coolant finding a port and blowing steam?
I've also had cars with bad water pumps lose coolant over long periods, but no obvious leak. None of it made it to the ground, mostly because the bearing and shaft was so hot it just boiled all the liquid out until it left a waxy substance that blended with the rest of the oil weeps. The pump finally totally died and once swapped the coolant has remained at a steady level.

Just some thoughts that might be worth trying to rule out for cheap/free. There are likely head gasket sealers that work, but I'd do everything to confirm the worst before heading down that path, myself.