Try pressure testing the cooling system- - - - -7-8 PSI is plenty, since that's what the truck was designed for. Leave the system pressurized for a few hours, then come back and see if the pressure is still there. If not, keep pumping it up and waiting. After a few tries, pull the spark plugs and spin the engine over with the starter. If coolant squirts out of one plug hole, you've found your leak. A slight compression/combustion gas leak will cause the symptoms you're describing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!