Over pressurized antifreeze tank

[HES]

Ok so yesterday I had this exact same issue. 2012 cm2250 in 2013 kw t800. Pulled into an oil battery to unload and low coolant warning light came on. Cooled it down and added coolant. Next load I pulled a hill and oil temp crept up to 220 so got unloaded and limped it home. This morning I put a pressure gauge on coolant reservoir and ran the truck at high idle. Pressure builds to 19-20 psi.Try new pressure relief cap, still builds to 19-20 psi. Try unhooking air line on compressor to air dryer, still builds to 19-20 psi. Combustion gasses getting into coolant reservoir? Not good. No visible oil residue in the coolant. When I went to add coolant this morning before starting the truck there was still pressure in the coolant reservoir not a ton but enough to notice it when removing cap. There is excessive blowby coming out the oil fill cap and dip stick. It seems like oil though not burning coolant (doesn't smell like it and when I hold my hand over fill cap it is oil residue on hand). The truck has used about 2L of oil since fresh oil change. I chalked that up to oil pan gasket that needs to be changed... I am thinking the worst here now. One month into ownership and i'm going to be inframing this thing. What is my next step? Pressurize coolant system and check egr cooler for leaks? Drop oil, remove pan and pressurize coolant system and check for leaks overnight? Send oil sample to confirm coolant in oil?

Any help from more experienced members here is much appreciated.

Truck History: I bought this truck a month ago. Did EGR tuneup on it and had SCR replaced under warranty/recall. 2 weeks later getting dpf codes and engine de-rate. Shop manager recommends I take to local shop that company uses to have de-mandated (programming done by DS). Shop manager offered to have company pay for it and I can pay them back over the next couple months. When de-mandate was done I told them to leave HP stock at 525. My thinking was this was my best option to keep my wheels rolling and then I can get set up with insite and have a recommended person on here look at the programming in the near future. I have purchased inline/insite from ebay link waterloo posted on the site somewhere but do not have it installed on my computer yet.