We have always left our gray open for months at a time while camp hosting. Never a problem with sewer flies or oders.. The hose is always clean also. We do not put any kind of food or grease down our drains. We take a paper towel and wipe down all dishes and silverware before putting them in the dish water. The only thing going down our grey drain is soapy dishwater, soapy shower water, soapy and clean washing machine drain water and water from brushing our teeth and washing our hands. All that soapy water does a great job of keeping the drain lines and sewer hose clean. You should never get sewer flies or oders in your camper because all drains have a drain trap that is full of water just like your home.
I don't see it as any different from my sticks and bricks home that is on a septic system. I don,t have a dump valve for my grey water on my home. The water goes down the drain pipe into the septic tank just like it does in the camper. If it was going to smell because of residue in the pipe or tank of the camper it would do the same in the house. No need to over think this.
