Put the chemical in the toilet right away with enough water to cover the bottom of the tank.
Don't drain the tanks until you're ready to leave. You can hook up the drain hose, but don't open the valves until you're all done using the facilities on that last day. Drain the black water first and then the gray water to help flush the black water. I like to do some serious flushing of the black water tank afterwards if the hookup is at the site to try and get it as clean in there as possible. After you're done, leave some water and chemical in the tank.
I'm learning myself, I'm on about a year and a half of using a trailer. My family is really enjoying it. It took me a trip or two to realize to fill the bowl with some water first before a BM.