Paccar mx 2010 epa not using def

I could use some help troubleshooting.

Little red light on def gauge started blinking and throwing low def fluid warning on dash. Plenty of fluid in tank. Discovered this is likely because it's not using def fluid.

Check fuses and harness connections, nothing evident there. About 650k on truck so i figure most components are original and likely due to be replaced. Start with what i can afford.

Had already replaced def pump filter when i recently replaced dpf filter.
Replaced doser injector because it had a good build up of def on it.
Replaced def pump, it was easier to do in short time then the inside def tank screen which i finally was able to replace also.
I could blow through all hoses, no restrictions or blockages.

Truck was intermittently flashing the red light and progressing to the 5mph derate (first I've ever seen this truck do such a thing) had codes cleared after trying something else for repair each time. Replaced in tank screen last and only got about half a day before the derate.

I have the short wiring harness and plan on throwing parts at it tomorrow. Seen some carton tops floating in the tank also do I'm going to just dump the def fluid to eliminate contaminated fluid. Aside from the main emissions harness I'm about out of ideas on what to do

 

No. I did drop the screen and fought hard trying to get it back until i learned how to remove the gauge and take the tank all the way off the truck.
Tank is mounted ps under front half of sleeper. Very difficult to work on it

 

Not really much from the fault list. I'll try to share pictures of most recent when i get parked.
Living in Casper Wyoming it's hard to find anyone able to legitimately diagnose. Have to basically burn a week to try to get into Kenworth. They help out allot by clearing codes and printing the faults at least. Hate going the parts changer way but I'm fairly limited to that considering income limitations and job demand

 

Think i forgot to say earlier that it did completely stop using def. That's what's been triggering the derate. It was getting codes for the def pump pressure. Didn't see anything concerning scr at all

 

Had to pull the abs fuse to get truck parked back at yard. Almost all of the codes are from that. Really makes it more difficult if the real problem is hiding under the codes from defeating the 5mph derate