If you're talking about extended stays at RV parks, you can reduce your propane use by installing an electric water heater element. (Or using the one already installed if available). There are a couple brands and they work great.
I installed one in my old '94 Bounder and in this '02 Journey it came installed from the factory. I haven't used propane to heat my water for over two years now, relying on the electric element and timing my shower and dish washing needs so they don't coincide. The factory installed heater is bigger than the aftermarket heater but the aftermarket element I installed helped a great deal too. Only required that the propane be turned on for 5 minutes before a shower and off immediately afterward.
If you leave your hot water heater on all the time on propane, that isn't necessary and wastes fuel. Just turn it on 10 minutes before you need hot water, and off after.
As far as adding a tank for your furnace, do your neighbors have one? Have you asked them where they got their equipment? That's the best way. Many towns you just call the propane company and they'll bring one out and attach it for you. You pay a deposit and go on a 'keep filled' month to month charge basis.
If you're talking about adding a tank so you can boondock longer, that's a whole 'nuther story.