Update and more urgent problem
So EGR problems corrected themselves and all was well for a short time. P21DD, bad resistance reading in DEF heater came back and wouldn't clear, then P20B9 showed up, so it really meant business. The heaters were back ordered at the factory according to local Chevy dealer so I ran it for a week or so waiting for it. The countdown started yesterday and today it went in and had the heater replaced, OEM one. Local shop did the work that I like. The codes cleared upon start up but the countdown continued. It was low (20%) on DEF so I added 2 gallons and drove it about 25 miles to hopefully clear the countdown BS. About 5 miles into the drive a regen started, with only about 10g of soot on the meter. Regen continuing, I am home now, with about 10 miles to go before limit to 55, then 75 miles until 4MPH limit = tow truck. There are NO codes on the truck, check engine light is not on, no P21DD or anything, after several starts and stops, including several ignition cycles.
Does anyone have any ideas? Off to dealer tomorrow if not.