For the gray water tank, leave it open to drain, nothing needed.
But I assume you're asking about the black (toilet) tank. Don't leave it open to drain or the liquids will drain off leaving a pile of solids concreting on the bottom of the tank. So, fill it and then open and drain all at once when it's at least 2/3rds full (or you're ready to leave). Chemicals are optional, but most use them. I use the drop-in campa-chem packets from walmart. Prime with your chemicals and a couple gallons of water before using.