We only use onboard water for showers and toilet. We use bottled water for drinking and cleaning dishes. We rarely use the water tank. I doubt the energy and money spent to prevent an attack to my water supply is worth it considering the minimal risk. I have never heard of someone contaminating an individual water supply.
Seriously, if someone wanted to mess with your water supply, they could simply turn off your fresh water spigot, remove the hose from the spigot, empty some water out of the hose, dump dangerous liquid into the hose, reattach to the spigot and turn the water back on. The dangerous liquid in the hose would then make its way into your city water connection and contaminate your supply.
If you are that concerned about it, perhaps camping is the wrong way to vacation. What about messing with your tow vehicle, what about removing your lug nuts from your camper? If someone really wants to mess with you, they are going to. Its exhausting trying to live in paranoia...