So the evap temp sensor definitely solved the problem. If I didnt have the scan tool to see what temperature it was reading, I am sure I would have wasted a lot of time and effort with the refrigerant side of things. This is because it presented as being low on freon or a clogged fixed orifice.
As to the crazy oil pressure readings, I worked on that as well. I got the truck from friends that kept it up and had all of the service records. Thats where I found that they had the original pressure sensor replaced at a independent shop and they used a NAPA brand part. From all my research I decided I should replace it with a genuine Chevrolet part, which is what I did. I saved the "old" sensor in the bag that the OEM came in. So I swapped out the sensors the other day and verified the screen was clean. That seems to have solved the oil pressure problem and I havent got a code back in over a week. Honestly after seeing multiple threads of people having the GM part fail in less than a year, I dont think I have much faith that its any better than a name brand replacement.
The only codes I have left are the ECU and cellular antenna. I am fairly sure the ECU code is due to the tuner deleting the AFM. As to the cellular antenna I think I was wearing my tin foil hat at some point while repairing the truck. I had the complete dash apart repairing the cracked parts and I think I disconnected the antenna so they couldn't track me! Either way these 2 codes havent affected how it operates and I am just going to leave them alone.