For the most part, there are two types of hot water heaters in RVs. There is the Atwood hot water heater and the Suburban hot water heater. Some coaches have the Aqua Hot systems, and that is a different set up than what we are talking about here.
There is one thing that any RVer can do to prolong the life of their hot water heater, and that is to drain and clean it annually. There is no need to spend the money to get this work performed at an RV service center.
These hot water heaters will always collect sediment from the water, and this sediment ends up settling on the bottom of your tank, and gradually will create a coating on the inside of the tank. If you let it get bad enough, this sediment can make your water heater less efficient, and can also be damaging to the tank.
The Suburban hot water heater has a steel tank and the Atwood has an aluminum tank. The way that you can tell what brand water heater you have is to go look at the hot water heater door on the outside of your RV, and if the vent screen is on the left, then you have an Atwood, and if it is on the right, then you have a Suburban.
My RV has a Suburban hot water heater, and I made a YouTube video of me walking through the steps of cleaning the hot water heater tank. The process is similar for an Atwood, except for one main thing. The Suburban has what is called an anode rod that doubles as the tank plug. Because the Suburban has a steel tank, it will rust out if there is not something in place to prevent this. Most anode rods are made of magnesium, and they actually sacrifice themselves so the steel tank can live another day (or years). In other words, the corrosion and rust occurs with the anode rod and not the steel tank. Science in action - very cool! Some put an anode rod in their Atwood heater, but it is not required, as they have aluminum tanks.
Whether you have a Suburban or Atwood, you need to clean out the sediment from your tank. To do this, you first need to shut the water off to the RV, then turn off the heating elements to the hot water heater (gas and electric). Next, you need to allow the unit to cool. If you want a quicker method, then turn the heating elements off, leave the water on to the RV, run your hot water in your RV until it is cool, andthen turn off the water source to the RV. Next, release the pressure in the hot water tank with the release valve, and unscrew the plug/anode rod and let the tank drain. Because of the design of the water heater tank, once the tank has drained there will still be a quart of so of water on the bottom of the tank, and this is where a good deal of sediment settles. Place a hot water heater tank wand inside the tank, and after turning the water on, twist it, turn it - really spend the time to get the sediment flushed out. You will likely be surprised at what comes out of your hot water heater. The tank wand is a great tool and is worth the small amount that they cost. Once the tank is clean, replace the anode rod/plug, turn the main water source back on, and give the tank time to fill. Finally, turn the heating source back on. It may take a bit for the hot water to start flowing back to your faucets, but it will get there; be patient. For those that have done this before, let this be a reminder to get out there and do it if your year is up. For those that have never done this, I would encourage you to give it a try; it is very simple.
If you are interested in learning more about the major systems in your RV, then you can always take the RV Tech Course taught by Terry Cooper, the Texas RV Professor. It was an excellent course and was very informative!