ISX-15 pushing coolant out of reservoir tank -help-

[Jeka711]

Hi ppl...

I have 2013 pete 386 with isx15. Im at 589k miles.
This trip i got low coolant light on, pulled over, checked and noticed the engine covered all in coolant.

I noticed my primary tank was half way empty and secondary coolant tank was full to the neck of the cap, so i figured the coolant is coming out from pressure cap.

I refilled it, was watching the gauge it was doing good but not for long. I was going to Ohio from CA and i took i-40 east. Lots of hills plus i was like 75k gross... i noticed every hill i climb more coolant comes out,

Started my shift end of NM and drove all day with no issues, coolant light never came on. The following day, same , no light, but i kept checking coolant and secondary tank was empty and my primary was full..

I added 1 gallon of coolant in 9 days so far. It goes away than comes back...

What should i check first? Could be my air compressor? Could be head gasket?

Never over heated my truck, and no DEF no egr no egr cooler... it could also be my spacer that been installed after the removal which ill check..

Just looking for sum feedback on how to proceed with this issue..

Thank you all in advance...