Burning Coolant

Um well are you selling it or keeping it. Company I worked for had a UD ( 900 International ) with an external coolant leak from the head gasket.
We sent it to the dealer. Picked it up and took it straight to the lease customer. Next service we found
the oil filter was the one we put on at the last service and the coolant was straight water.Called and asked in disbelief that they did a head gasket and did not change the oil. Trash getting in the bottom and such? That is not covered by warranty! What about the coolant? That is not covered by warranty! It had antifreeze in it when it got there. It was clear we were done with that conversation.
Let them do the deal and sell it or pay extra for an inframe while you are there. My 2 cents and hope you have better experience than the UD.

 

Most common leaks are out of hoses if none visible the only way it'd be using all that coolant would be it there were some cracks in the engine block although you've said there's not sign of water in the oil which is usually the case if water is seeping into the block housing. Its done over 750,000 miles so there is a real possibility there is a crack somewhere in the engine block and water is getting through someplace.

 

Most common leaks are out of hoses or radiator if none visible the only other way it'd be using all that coolant would be it there were some cracks in the engine block although you've said there's no sign of water in the oil which is usually the case if water is seeping into the block housing. Its done over 750,000 miles so there is a real possibility there is a crack somewhere in the engine block and water is getting through someplace.

 

Put a new turbo on, put things back together, get her back to work and find a new owner. And for you a new ride.
I know it's easier said than done.