Isx15 popping radiator caps

[FnG92]

Hey guys I've got an isx15 2350 mandated and about 2 months ago I popped a radiator cap. I replaced it and have been keeping an eye on it. The last time after blowing the first cap I had to add coolant and then I was having def quality derate so I changed my def level sensor assembly. I ended up removing my def heater coil so I disconnected the coolant lines to do this and lost coolant about 3 weeks ago. That's the only time from popping my cap the first time 2 months ago that I added coolant. My overflow side of my coolant tank has a small amount of coolant in it. Truck has 335k miles 2015 t800. I hope it's not the head. I can't afford an 8k head right now.
I drive the way rawze preaches but do not have a Pyro gage. Truck is showing 7.7mpg and it's historically been within .2mpg. Half of all my miles are empty while the other half are 80k gross. When I bought this truck I was getting 4.9-5.3 mpg. Nox sensors dpf filter doser injector and I'm starting to get good mpg. I wouldn't be left standing without the help of rawze and everyone here that shares your knowledge and experience.