2017 International ProStar N13 - Coolant disappearing, no external leaks

I have an N13 that is eating coolant. There are no puddles under the truck, the oil looks clean (no milkshake), and the surge tank isn't showing any bubbles like a head gasket. However, every morning I have to add a gallon and there is a thick cloud of sweet-smelling white smoke for the first few minutes of idling. The truck still has plenty of power, but the smoke is getting worse. I'm terrified it's a cracked liner, but I'm hoping it's something less invasive. I did a pressure test on the cooling system and it dropped about 5 psi over an hour, but I couldn't see where it was going.