Loosing Coolant slowly from top of the tank.

1 - What comes out of your air tanks when draining? Check the air compressor output line. Remove it and see if it is dry or if there is coolant there. You can also run the engine with the line off. Combustion can get into cooling system through the air compressor 2 - cracked engine head or blown head gasket 3 - leaking EGR cooler though more likely to only lose coolant with a leaky cooler.
With Cummins the coolant comes in direct contact with the cylinder liner and if pitted bad enough combustion can get into the cooling system that way.

 

Could be the plastic coolant tank. I want to get an aluminum one l have seen online.

CUSTOM Aluminum Coolant Reservoir For Freightliner Cascadia Columbia 2008-2017 | eBay

 

Clean that off and inspect that surge tank. It may be cracked.

Once they go, the radiator will follow within a week or two.

They both are very thick plastic. Radiator is thicker.

After so many years, they fatigue from expanding and contracting.

The surge tank goes first. Then after you replace it, because it's new and very rigid, the fatigued spot(s) in the Radiator go.

 

Possible, but look closely at what he said. And yes, check the simple things first.
"...coming from under the top cap"
"the overfill portion of the tank on the left bottom was empty. Not anymore."
Sounds like excessive pressure in the system.