That model has small tanks to start with
http://www.keystonerv.com/previous-years?brand=Outback&year=2005
Apparently both the black and grey tanks were 28 gallon, so on the grey in particular you will need to dump often especially if you are not being overly conservation minded.
If you are able to stay in place and connected just leave the grey open until a day before you need to dump black. Then dump both. A weekly schedule will simplify it some, but make sure you store grey for a day each time so you have a volume of water for clearing the black from your hose.