If you're really lucky, you could just need an air compressor. You would probably already know if it was putting antifreeze in your air tanks. It can also leak antifreeze into the oil. If they pull the pan to check for leaking liners, they'll check the compressor too.

Of course, if you have anitfreeze in your oil, there's a good chance you could have bearing damage.

 

Antifreeze like oil finds it's own level.
Keep some on hand but don't add any just to see what happens.
Take a clear gallon jug and fill it with water and put your rad overflow hose in the jug with the engine running.
Any bubbles?
Another thing to check that has fooled many people is the small 3/16 line that supplies air pressure to the bunk heat/air switching valve.
For some reason that line is at full system PSI and the valve will bypass air into the coolant causing the system to overpressure.
There is no shutoff to the supply but if you take some vice grips and put something on the line to protect it then pinch the line off and check the overflow again for bubbles.
Before getting too excited about head gaskets etc. I'd try that first.
That's happened to me three times and I know of cases where dealerships have done the heads.
But then again if you have warranty and they want to do the heads.

 

I used to drive a 3406 a few years ago. Got to a delivery in CO and noticed a "wet line" on the ground as I backed into the dock. When I checked it out, the radiator was leaking out the overflow. It was fuel. The injectors seals in sleeves with o-rings. one of the o-rings broke and it was pumping fuel into the coolant. But, depending on which ring fails, it can also pump coolant into either the radiator or into the cylinder. A gallon a day is a lot to lose.

 

When the engine is running, the fuel is at a higher pressure than coolant... therefore you get fuel in the coolant, causes overflow (with a fuel smell). When the engine is shut down, residual pressure from the cooling system overcomes the now "near zero" fuel pressure and pushes its way into the fuel --- Coolant in the fuel can be devastating to fuel system components, especially fuel injectors. It is quite unlikely for coolant to get into the combustion chamber from the injector sleeve... it actually gets there indirectly from the contaminated fuel in the tank.