Our 2016 Minnie Winnie 27Q has a P-trap on the lav sink. I'd first disassemble the P-trap to see if there is a blockage or maybe a restriction caused by a plug of hair/grease/soap.
If no obstruction is found, I'd replace the under-sink air vent. The thin rubber diaphragm inside it can and does deteriorate over time. It will be just under the bottom side of the countertop and possibly under the sink itself.
Replacement air vents are available at Home Depot for around $10.
2016 Minnie Winnie 27Q on a 2015 Ford E450 chassis. Retired U.S. Air Force. Living in Anchorage, Alaska