Problems with sewer gas being sucked into the trailer from the AC

Hi, we have a new Elkmont. We are camping at an RV park in the heat of Southern CA. Whenever we use the AC, it seems to suck the sewer gas from the toilet roof vent into the trailer. We have cleaned the toilet tank and refrained from using it. Also, we are hooked up at an RV park and there is no sewer gas coming out of the in-ground hook up. The only thing we can do to eliminate the odor is to fill the holding tank with water so that there is no space in the tank for gas to be vented to the roof. The fundamental problem is that the roof vent is located immediately behind the AC. In houses, there is a building code to prevent this problem by requiring that air intakes be located substantially distant from such bathroom vents. Has anyone else experienced this problem? And if so, how did you fix the issue? Thanks so much.