I will assume you have a water supply and not using your FW tank. As long as you dont take long showers (do a navy shower here) you will be OK. I boondock all the time and easily get a week from my gray tank and a month from my black tank..
2004 Chev Silverado Duramax optioned past the max. 2009 Jayco Eagle 308 RLS 765 watts of solar, 6-6 volt batteries (696 amp hour), 2000 watt (4000 surge) whole house inverter.
140 days boondocking in 2016
211/2015, 196/14, 247/13, 193/12