ISX450 coolant issues

[Chubbs68]

Evening everyone I have 13 386 that I have always smell coolant but never find at one point the coolant went brownish black then last year head gasket went out decided to do a in frame 600 hp upgrade. Still could smell it would lose about a gallon between services (10,000 miles). I mainly smell it on the passenger side under door and inside cab but no coolant inside cab so assuming heater core is good just did a service and coolant is getting dark again it doesn’t smell burnt my mechanic is baffled anyone got suggestions . Thanks all