Sounds like you are referring to the black screw-in Air Admittance Valves located after the "P" traps. These are used to prevent vacuum in drain lines so the 'P' traps don't get emptied when you dump the tanks.
The valves could be sticking, I guess one could unscrew them and try soaking them, but they are fairly inexpensive item to replace.
Also, check that your roof stink pipes aren't blocked.
Hope this helps.
Bob
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2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (Strait-Line)