Lets take this step by step.
Do you have water in the fresh water tank?
Do you have the switch for the fresh water pump turned on?
If all else fails, you could take a bucket and put it under the fresh water tank drain (probably under the rig), to get water for flushing and hand washing.
It would help other Sierra owners if you posted year and model number. Its not listed in your profile. Pump locations vary by model.
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package