The problem is your air admittance valve under your sink. The cheap black ones used by all RV manufacturers fail way before a good one will.
Remove the black air admittance valve by turning it counter clockwise, then install a new 1-1/2" air admittance valve. There are several brands, but Studor Vent is the most used in the residential plumbing industry. You can also find them made by Oatey, ProFlo and others. Do not buy another cheap one. You should be spending about $20.00 on a good one.
This is going to fix your problem.
2016 Rockwood Windjammer 3029W Diamond
2015 Chevy 3500HD LTZ 6.0 Crew Cab 4x4 Long Bed 4.10:1 SRW