All the cities (or at least most) have public fountains. There is even a mobile app that lists them : Eaupen . A good plan also is to think to ask petrol stations when refueling, if they have a tap. On construction sites also, by asking workers.
To drain waste water (black for WC, gray for the shower water, Sink), there are lots of free drainage points (sometimes pay : 2 euro as the Flow Blue Relais), indicated by the EC24 panel. L’application park4night I have already cited for parking for each location indicates the services provided, on their website bottom of the page you can click Without stat service area. then select a region, the department and you have a list of addresses.
About WC, We plan to test dry toilets, bucket, a bag and hop in the trash as dog excrement or diapers !