The driveway campout test is the best way to figure it out.
I use real plates, etc. I wash them with my fresh water. I have yet to run out of water in 4 days (I'm usually about halfway down without being frugal). I cook, clean and drink from my fresh tank.
For battery, I have yet to drop below full green. I use lights when needed for cooking, etc. I charge my cell, kindle, etc., as needed. My fridge is on propane (and is old so doesn't use any battery power). I have the water pump running the entire time.
My propane lasts forever and I have my water heater on throughout the trip.
So, it's really subjective as to what you will need. Testing your own usage will prevent you from buying unnecessary items and/or having to be more frugal than you need/want to be.