No experience on a Detroit, but almost every time I've seen this happen on a Caterpillar, and everything else has been checked/changed, I remove the head and carefully inspect all 6 fire-rings on the old head gasket. The fire ring is the solid metal ring on the inside of each of the 6 circles. (A lot of shops pitch the old gasket without inspecting it). There will be a small black "soot" mark usually about 1/16" inch in one or two corners of one, two or three fire rings. That soot is where the combustion is leaking into the cooling system. (If that's the problem). It will at least lead you to inspect that particular spot on the deck or head for abnormalities.
My '01 DDEC 4 literally goes through 0 coolant at all. The only time I've added coolant is after a hose repair.
Another way to verify if it's combustion gas is to make a home made radiator cap (I used an adapter from an old cooling system pressure tester) with a manual pressure release valve on it and a 15-20 lb pressure gauge. Run the engine to operating temp-bleeding off the air pressure about 4 times to remove the natural pressure formed from coolant expansion- then see how high the gauge will go (Don't go much above 15 psi or hoses may start blowing). With the problem as bad as yours, I would do that with the air compressor completely disconnected, tanks drained and heater valves all off.
If it still builds pressure, then you will have 100% confidence inspecting the engine again for the combustion leak.
They can also pressurize the radiator (engine cool) to about 10 psi, then remove the oil pan, all 6 injectors, and any cover on the front of the engine and search for the coolant leak with a high quality borescope. Even if it takes all day, a good, detailed search will locate the coolant and then maybe lead to your problem.
I'll let you know if I come up with anything else.