losing antifreeze

What is the model of truck make an engine type

 

are you getting white smoke out of the exhaust? That is alot of coolant to loose and if there was an external leak I would think you would notice some residue somewhere. I'd fill it backup and let the truck run for awhile and watch for leaks.

 

If you find the cause, let me know. My '05 MXS 550 goes through between a quart to a half gallon a week. No leaks, not going out the overflow tube, so my guess is it's burning it. I have 2 1/2 years remaining on a Cat OPT overhaul and neither Cat or Peterbilt(where the overhaul was done) will do a thing about it. They tell me they have to have a "real" reason to tear it apart. If they don't find anything then it's my bill to pay. Some warranty! Since it was gone through last July,(6 months after the original overhaul, cracked head on #1 cylinder and head gasket on #5 cylinder) I've gone through over 15 gallons of extended life coolant! But there's no problem! Sorry for the rant and hijack! I hope you find your issue.

 

Check your oil and make sure it's not milky looking. You may have coolant leaking past a cylinder liner seal into the oil. Could also be a blown head gasket. With the cap off, look into the coolant reservoir with the engine idling. If you see the coolant bubbling, then you've likely got a blown head gasket.

 

Burning coolant will show up in an oil analysis as a high sodium level.

 

Have you checked to see if it is getting into your fuel or oil?

 

Burning up in the EGR cooler?