2007 ISX CM870 Dirty Pre-DPF in a CC Magna.
We're researching the same kinds of coolant lost, but not as much of an amount as you are experiencing. And have detected no coolant odor or white smoke in the exhaust.
A few other tips to check:
-Radiator Cap (Ours was showing signs of rubber rot in the gasket, so replaced with a good quality 16lb radiator cap.
-Tied to the Radiator cap, is the hose overflow. Tape a plastic bottle to the end of the hose, and look to see if you are seeing any coolant build up in the bottle. (This, along with the mentioned above head gasket (And other.) leak, could be pressurizing your system.)
-Check for hose clamp leakage. (Crawled under out engine area with a strong light, telescoping mirror, and ours all seem to be dry.)
-Open your front generator slide out area, and look for sings of leakage from the OTR heater hose run.
-School Bus Pump (Which I could not get to when I crawled under, due to the chassis was too low. Will put the drive axles up on blocks to inspect the School Bus pump for leakage. And will bypass if it is.)
-Gary Obermire, Premier, also suggested I pul the Trans Dip out. Transyn should be clear, if you see signs of clouding, or two different coloration posts on the dip stick - could be coolant merging with the trans.
All that being said. I've made an appointment in late August for Cummins (Colburg), to help me look over the engine. I'm bracing myself for the above mentioned EGR leak... I will only be driving the coach about another 300 miles before then.
Please close the loop back when you find out the root cause, and I will too. And best of luck to us both