We got sewer smells in the bathroom so I popped the covers off the vent pipes and found that the pipes had been cut short and at somewhat of an angle. The gap where the pipe penetrates the roof was not sealed. It seems that gases could get into the ceiling space. I sealed the gap with silicone. We haven't been camping since then so it's wait and see if that fixed it. If that doesn't do it I guess the next step would be a Cyclone or 360 Siphon vent cap for venturi extraction.
2015 Microlite 21 DS
2012 Silverado extended cab Z71