Sewer smell in cabin - Event Date: 26 Jul 2016

First question is do you have the sewer line connected and is the gray tank valve open? One observed problem is a bad Air Admittance Valve (AAV). This is a device associated with each sink (two per Pod) to prevent emptying of the P-traps. A bad AAV will let sewer gasses from the campground sewer be sucked back into the Pod if the gray tank valve is left open.

Another cause is the fan sucking bad air directly from the black tank when flushing, but this is only transitory. This is worse with the Pod windows and door closed because the fan will suck from any available air source. 

+1 to what Charlie said.

Most likely, there is not enough air coming into the pod through windows, so the fan is creating a negative pressure. When flushing, that negative pressure pulls from the black water tank. The black water tank has a vent on the roof that is supposed to vent outside the pod. However, with negative pressure, air is pulled into the vent, through the tank, and out the trap along with the odor. Turn the fan off temporarily, and no smell. Note that even if enough windows are open, the openings at top and bottom of the door are not enough to keep the airflow at the pace of the fan, especially if it is set to anything more than the lowest speed. You can demonstrate this by watching the fan pull the bathroom door shut if it is only open a little bit. If the shower and/or sink trap is dry, sewer fumes can be pulled back in there as well. Make sure there is water in the traps. If you don't use the shower, it is likely dry. Pour a couple of cups of water into the drain to make sure it has water and will stop the odors.

Also try using a liquid sewer treatment. I've found the blue aqua kem stuff helps. Powders not so good. I do one bottle at the start of each trip and again after I dump (unless heading home).

Even if you add water to the shower pan drain and it solves the problem, add it every trip after arriving.  We've found with our current camper the water sloshes out bouncing down the road.