Truckcamper nomad

I am on a trip now with my 2021 6.6L Duramax. Got the warning that my def was nearly empty although I had filled it about 300 miles earlier. I added about 1 gal and ran it over. I then watched my DIC gauge and it bounced from nearly empty to nearly full and everywhere in between over then next few hundred miles. We are at the beginning of a 3000+ mile trip so I can’t run to my dealer! I will call a dealer in Moab on our route tomorrow and see if there is a quick fix. Hope I don’t get put in limp mode! Any advice out there?