We have seen a lot of times that the water heater itself is not even plugged into an outlet.
You may have to remove a false panel, unscrew a fake hinge, go thru a cabinet, or other to access the backside of the water heater. Once you do access it, the first thing to do is to find the electrical cord for the water heater and see if it's even plugged into an outlet. (or connected to an electrical circuit)
On many RV's (not all though), it will require access to the backside of the water heater to get to the water heater bypass valves for winterization....... so it's a good idea to know what you must do to access from the inside of the RV anyway.
I am assuming you have a Suburban water heater as you indicated there is a switch (inside) the heater, but think you mean on the face of the heater. If this is correct and you have another 'inside the RV' switch for the electric side of your heater..... yes there is a relay. I just posted a file for this type of Suburban the other day in a separate thread. Let me see if I can find it. I think the relay part number is 232948, which I have written on a pad still in front of my computer.
And yes, it does seem your breakers are labeled wrong. My RV has a 20 amp for the air conditioner and a 15 amp for the water heater circuit. Mine is very similar to this google search pic.