Something else not mentioned. Whatever you do as a temporary fix, run your heater the whole time you are driving. The reason is that the way the circulation works, you loose the water in your heater core long before your temperature gauge will ever show you are getting hot. This first sign is at idle, the heater will go cold. If this happens, you have not damaged anything, but you are beginning to run low. Eventually you will loose the heater at speed. At that point, you need to find a safe place to pull over and refill. This will give you plenty of warning long before any damage is done.
I should also mention, make sure you have hose fittings, so you can hook up a hose directly to one of your faucets and make sure the hose will reach your radiator. You don't want to be fumbling around with filling pots and pans and pouring it into the radiator. In other words, make sure your fill method from your fresh water tank is quick and easy. You don't want to be sitting on the side of the road messing around with it.