The temperature doesn't drop. Coolant stops flowing because it boils, and the temperature gauge is FOS because it isn't immersed in coolant. ANY unusual temperature reading at the gauge is cause for going over the cooling system.
And FYI, absence of baby barf in the crankcase does not rule out head cracks. It only rules out really big ones. Condensation is a likely possibility ONLY if the truck is driven only very short distances, to prevent full warm up of the oil. Get it above boiling there (which is very easy), and it will slowly purge out the PCV system.
Mystery coolant disappearance does suggest a head crack, but double check the water pump first. And just to be sure, leaks are NOT identified by puddles. Only really big leaks make those. They are identified by crusty deposits around the hoses, core plugs, radiator core, heater hoses, and so on, and ESPECIALLY corrosion tracks on the bottom of the water pump. Everyone seems to miss that last one.
If you are BURNING coolant -- which makes persistent sweet-smelling white smoke even when the engine is completely warm -- you likely have a head crack, but can easily confirm it by pressurizing each cylinder to 100 PSI at TDC/compression with the radiator cap removed. If it's blown, you'll see the air coming through the cooling system. There are no in-situ tests that can distinguish a head crack from a blown head gasket (excepting a borescope with a crack big enough to actually be seen), but either requires removing the head in question, and it can easily be Magnafluxed then. The pressurization distinguishes a blown lower intake gasket.
Note that head cracks do not have to enter the cylinder, and mild ones often don't.
The thing that nails people is that the vehicle can run nicely with a mild head crack, until it starts to eat the crank and rod bearings, or until it overheats enough to "flatten" the pistons. At which time a $500 repair has turned into a $2000 repair. By the time this happens, the engine will make all kinds of periodic knocking sounds, will start smoking (burning oil through scored cylinders), and may seize or break.
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