When my RV is not being used, I have found it advantageous to leave the sewer open and the cover off to allow the tanks to be vented. I was concerned about insects or whatever getting in the tank.
Using a couple of cable tie wraps, or a hose clamp, I fastened a small piece of screen to an extra sewer hose connector and this has worked out great.
If you found this tip helpful, find more great tips from RVers like you in Ten Minute Tech, Volume 3