Is the reservoir supposed to remain full when the truck is driven and lower when cool? My coolant res is full and in the mornings it is back to half. Any suggestions why?

Or am I missing how this all works?

 

That is the correct behavior. As the coolant gets hotter, it expands. Radiator cap only allows about 16PSI in the radiator, beyond that, and it will vent some coolant to the reservoir. As things cool, coolant contracts, and gets sucked back out of the reservoir.

 

This is why you shouldn't open a radiator cap when the engine is hot. ESPECIALLY if it's overheating. The release of pressure can cause the coolant to flash to steam which is dangerous as holy hell! Last year at one of the mud runs a guy was adding oil to his truck when the upper radiator hose blew in his face, it was already running hot and since he was running straight water the engine temp exceeded the boiling point of the water and the sudden release of pressure when the hose ruptured caused the water to flash to steam right onto his face and chest. He left the place in an ambulance.