where 2 get water for RV before you reach your destination?

I have never seen an established campground that didn't have water available somewhere on the property. If you're rustic camping or boondocking all bets are off.

Rest areas in many states.

A couple of options we have used.
Sani dumps
Because most places that offer a dump station also have POTABLE water available in the area.
and these come in handy for places that a faucet does not have the threaded fitting for attaching your hose.
Water Bandit

Most campgrounds have water filling stations ( if you didn't get a site with water on it), depending on what time of year you are visiting. Some you can use you hose with, others need a water bandit. This is mostly area dependent. Or if the threads are broken.