I have a 2020 V-6 3500 (it is a 2021 Winnebago Vita RV) with 12,500 miles, it tripped the CEL a few weeks ago.
Took it to MB dealer and they diagnosed it as needing new NOx sensor, ETA on part is January '22.
So I bought a cheap ($27) OBD code reader, codes are "P229E" and "P229F". I clear the codes after each time CEL comes on (which then turns off the CEL light)... I have not gotten the dreaded countdown timer, hopefully I can manage that by clearing the codes...? Fyi, CEL comes back on after driving about 30 miles or so.
Prior to the CEL issue showing up a few weeks ago, I went on a 9K trip cross-country, averaged 14 mpg and was impressed with the MB part of the RV, but the emissions system seems like its going to be a very weak link in an otherwise very capable van.