2017 Def gauge stuck at 1/2

Just bought a 2017 Sprinter based motorhome. The def gauge was at 1/2 so I added 2.5 gallons. Drove 75 miles or so and the gauge is still at half. No check engine lights. Def tank is definitely full.

I'm leaving on a trip next week. Do I need to worry, or is gauge just slow to update? I've read that draining, filling with distilled water, draining, and refilling with fresh Def might help. I'm not sure how much the motorhome has been used in the past year.

I'm ok if gauge stays at 1/2 if I don't get a 10 starts countdown.

Any advice?