You can get a fresh water refill from the guys who service the porta potties. I know people who have done it.

But at two gallons per day, it seems easier and a LOT cheaper to just carry some water with you.

Personally, I carry 80 gallons in my water storage tank. That is reserved for showers and other cleaning. I reduce water use by using disposable dishes. Dishwashing can be a relatively large water consumer. All of our drinking water is from bottled water that we carry with us.

My holding tanks are 40 gallons each. Black storage is not a problem. Plenty of capacity for that. For the gray water, I build an 8 foot square frame from 2X4's held together with angle braces. I staple black plastic on top of that to make an evap pond. I put a block in the center to keep the plastic from tenting up in a high wind (this might be overkill), and I tie the 2X4's down by pounding L-shaped rebar through holes pre-drilled in them. I put solar powered lights and a flag or two up to warn pedestrians of the hazard.

The entire thing takes about 15-30 minutes to assemble.

Each morning, I push the accumulated dust aside in order to expose the heat absorbing black plastic, and I let a few inches depth of gray water into the pond from my holding tank. I stop doing this two days prior to departure, to be sure that all water evaporates and the plastic is dry for departure. When it is time to go, I just roll the plastic up and stow it to be discarded later.