A big problem trucks have been having with a wonky def gauge reading, this may be telling The computer you are low on fluid but really you are not. The level may not be registering. Not sure if you have tried this.
from under the def tank. Unplug the wiring harness from the pump/level unit at the bottom of tank by base plate. If that makes sense. Start the truck which will generate a check engine light. Stop the truck clear the codes with a scanner (all that you can) Plug the wire harness back in, check the connection. Clean and grease it. Need to make sure you have fluid in tank after.