Get the sewer hook up and practice. Gray tank can stay open until the day before you are ready to dump the black. You're not likely fill the black tank in 3 days unless it is very small. You want to collect some gray water before dumping the black tank to rinse out the line afterward. Whwn I ma camping for a week I leave my gray open most of the time and the black closed until it needs to be dumped. You could fill a small gray tank in a day if you do not conserve! Dumping the tanks at the site is a great convenience.
