You probably do not have a water heater. You probably have AquaHot or HydroHot. Most are on the passenger side, in a dedicated bay.
If you validate you Aqua/Hydro Hot, look in your manuals for recommended servicing. Usually a new novel, filter, and cleaning of the combustion chamber are all that are needed. Radiator caps are cheap, so replacing them every other year is not a bad idea. Be sure to get the proper pressure cap. Finally you should find a expansion tank, marked with high/low levels based upon temperature. (Hot when the unit has run, and cold when it has been sitting for some say overnight without usage.) If you do not know the history on this one, assume that it is OEM and has never been changed out. This is boiler tank type stuff, manual will tell you for sure what to use. Our's is a reddish color, and I use Camco. A knowledgeable HydroHot Tech advised me that most people keep topping off with more of the Camco Boiler Tank fluid. He suggested that I used distilled water one year, to help dilute the too heavy of concentrate of Boiler fluid. Then the next year use the Boiler Tank fluid to top off. He said that when the fluid became to heavy of a concentrate of Boiler fluid, it does not exchange the heat as well with the heating coils. So, I've just started this year to change things around to every other year to topping off with the distilled water. (I buy the Camco and or distilled water by the gallon. Put velcro on the bottom of the bottle, and then in my bay I have enough room to one side of the Hydro Hot, to place the other strip of the velcro. This keeps it from sliding around and bumping into things, and preserve storage space in the other bays. Also makes it easy for me to remember which cycle I'm on. After my yearly maintenance, I just switch the bottles around next to the Hydro Hot, and know for sure which one I'm using that year
Mine may add 2-3 OZ every 2-3 months of usage, while traveling.
Best of luck to you