2010 isx burning coolant?

[Dmurphy16]

I’ve got a 2010 t800 with a Cummins isx. Bought the truck a few months ago but it’s been sitting. Finally getting around to getting it roadworthy. Took it out for a short ride about 20 min. While driving it was puffing white smoke, and the coolant level alarm came on. When I got back the coolant reservoir was empty, so I added 2.5 gallons or so. Took it for a ride again and same thing. Coolant is getting into the exhaust and being burnt. from some quick research it can be many things; head gasket dropped liner, egr cooler.
Service records show the truck has had coolant loss issues in the past, it’s been pressure tested, no loss, ruled out internal/external leak. Radiator has been replaced.
I am thinking it’s possibly EGR cooler? I would like to start trying to diagnose so I can figure out if it is the cooler I can send it to the mechanic to replace it, but know it won’t be cheap.
any suggestions?