It's possibly coming from the UniGard Waterless P Trap under the shower.
We have a 2021 3121RL and had the same issue. Traced it to the shower drain. Got a new trap from Etrailer.com. Very easy to install. Just had to pull back the curved wall behind the wet bay to access. Took 5 minutes to install.
Problem solved.
https://www.etrailer.com/RV-Sewer-Ho...l/LSB24FR.html
Paul