2007 and a half 6.7 Cummins
 

what does it mean when the reserve antifreeze tank is bubbling before the truck is warm up

Do normal temp all time or overheat?

Blown head gasket. Or leaking head gasket. What mods do you have. I bet you radiator is pressured up as well. Let all the pressure off start the truck see how long it takes to gain pressure if it gains quick it's the hg. Also remove the cap start it look inside for bubbles in the coolant