Something else just hit me too..... this one isn't strictly required but some people choose to do this... and that is to plumb your water heater with a bypass. This will take three ball valves, a couple of T fittings, and a couple of short lengths of PEX.
Cut about a 6 inch section of the input and output lines from the water heater in a location close to it (5 to 6 inches away), and connect a ball valve to each piece attached to the heater. Attach the pieces you cut out of the lines to the other side of the ball valves. Now insert one side of the straight-thru legs of the T fittings into the end of the piece you attached to the ball valves, and reconnect your water lines to the other side of the T fitting. Take the other pieces of PEX tubing and connect them to either end of the third ball valve, then insert that assembly between the two remaining T fitting connections and connect them. Crimp everything down and test for leaks.
The main advantage of doing this is, you don't have to shut down your entire water system if your water heater springs a leak. Close the two ball valves on the input and output sides, and open the bypass valve. That way water can continue to flow through the hot water system, even if it is cold water. A second advantage is if you park your rig for the winter and choose to fill the water lines with RV antifreeze you won't have to waste 6, or 10, or more gallons of antifreeze to fill the water heater. Bypass the heater like before and just drain the heater tank. Even if some water remains in it, it will have plenty of room to expand if it freezes.