Air Gap Vent

Directed to those of us with a rear kitchen model. FR has put an air gap inline between the sink trap and the vent to roof pipe. It is now beginning to smell ( a sewer smell). Has anyone else had this problem? Time to replace it but where do you get this part? Why would they install this to vent into the trailer?

They sell them at ACE hardware. It's not really between the trap and the roof though. There's another vent that comes directly off tank and either goes through the roof or opens into the top of the fridge compartment like mine does. The reason they use the air thingy under the sink is because its sometimes not possible to run a true vent up to the roof in a wall or something.