Replenishing Fresh Water Tank
Camco Water Bandit
Maybe you'll use this; maybe you won't. But if you ever need it, you'll feel like a fool for not spending the few dollars to have one on you. Besides getting water from spigots that don't have a threads to attach to your water hose, there are plenty of potable water hydrants out there that have stripped threads - so even with the proper threads they still won't attach to your water hose. Grab a hose clamp that will fit it too just in case.
Aquatank2 Water Storage Bladder
Anecdotal reports from RVers lean heavily toward this option. It's collapsible, so it only takes up as much space as the water you have in it at any given time. At a campground where you have an electrical hookup but not water, this thing lets you keep potable water in your fresh water tank while keeping your RV set up where it is. While boondocking (securely - see warning below), you don't have to break camp and haul your big RV to every little potable water supply, some of which are in places a big RV can't fit or where it would just be awkward.