Our coaches, and possibly all motorhomes, often don’t meet the same plumbing standards as those found in most jurisdictions. The air admittance valve Tom mentions is a case in point. Most plumbing codes permit their use only under strict conditions because, when they fail (which they inevitably do), they allow sewer gas into the living area. Our vent systems probably wouldn’t pass a local plumbing inspector’s scrutiny.
As Denny points out, keeping all traps filled can help reduce sewer gas presence. However, the air admittance valve, often used at end points like kitchen sinks, must also function properly. If the spring or check valve fails, it needs to be replaced.
The drainage systems in our coaches also fall short of code compliance, with several potential cross-contamination points—one of the biggest concerns in plumbing. Sometimes converters create opportunities for cross-contamination, like in holding tank clean-out provisions, but there are less obvious risks of fresh water contamination. This isn’t meant to alarm but to encourage everyone to familiarize themselves with their coaches and be aware of where converters have installed backflow prevention measures, such as check valves. These devices, like the air admittance valves, should be checked periodically to ensure they’re functioning correctly. Unlike the odors from a faulty admittance valve, cross-contamination may not be as apparent.