We are towing our Grand Design 5th wheel with a 2020 Chevy Silverado 2500 Heavy Duty Diesel and we received a DEF Fluid warning for 1000 miles. When we check the level it is at 3/4 full. The mileage does not seem correct because over the winter we would fill up with DEF Fluid and it would say 600 - now that we are towing we fill up with DEF fluid it says 360. We are thinking it is because we use more DEF fluid when towing so it throws the formula off and gives a warning. BUT if the truck is going to regulate speed because of thinks it is low we are out of luck. How do we move forward, the truck thinks it is low. The fluid is in there. The system is going to take over. It’s like the brain doesn’t the bladder if full.