P-Trap in Sewer Hose

Here's an idea I appropriated from a commercial campground. To prevent sewer odor from being sucked back into the camper from a campground sewer system while the gray tank valve is open and the fan is running, simply form a trap in the sewer hose. This allows the gray valve to be left open while hooked to the sewer system without any odor problems. Works like a champ.