'07 Freightliner blowing white smoke from exhaust and leaking coolant...

Could be both, you can pressure test EGR cooler easier then head so I would start there first. Not actually white smoke if EGR it is steam.
Test this by turning engine off, let it cool down, remove EGR valve (looks like a small beer can with wires). if is wet inside, steam cleaned, then it is EGR.

 

oops yet send to fast;
head gasket will smoke, steam, and 85% of the time pan will have milky oil. 75 bucks will tell you about oil if not visible.

 

to test the head gasket, take a bottle and tape it to the radiator support, and stick your overflow tube i the bottle and drive it. if the bottle has coolant in it then most likely its the head gasket. or you can get a bottle of water and stick the tube in ad see if it make bubble in the water bottle.

 

to test the cooler, try and take one off the boots off the charge air cooler tube, and look inside to see if it wet, or if there is like a white residue on the inside.

 

Then it might be the egr cooler

 

Can not this same scenario be caused by a bad air compressor?
Or the system could have been overfilled with coolant and the extra is being expelled as the coolant heats up and expands. Overfilled system not in this case.