[DutchmenSport]

An outside vent will smell, if that the be on the roof or the side of the camper. The purpose for the vent is to prevent creating a vacuum in the holding tank. Air has to breathe in and out of the vent, along with that comes the odors. If the holding tank hasn't been flushed out in a while, food particles stick to the sides of the tank and after a short while can emit an odor that is worse than the black tank.

Sometimes these vents are located on the roof near the ceiling vent fans. When the wind blows the right direction, the odor coming from the tank vent will be pulled right back into the camper through the fan exhaust vents (mostly in the bathroom is seems to happen most frequently).

You did the right thing by flushing the tank out. When we travel, we always try to travel with a small amount of fluids in each holding tank, (like around 5 gallons), so when driving, the movement of the trailer sloshes that fluid around. It helps to pulverize the inside of the holding tanks and break free a lot of that build up on the inside of the tank and will flush out the next time you dump.

If you do that every time you move the camper, your tanks will stay much cleaner. But, occasionally they do need a good soap / bleach bath.

Glad you've got your problem solved.