Now on the switch:
You stated this RV is a new to you but a previously owned RV. There can be several things that the previous owner may/may not have done, so you may need to look for some stuff.
The outside electric heating element switch on the Suburban water heaters, can be kind of confusing. To turn it on, you press the switch in where it says on, and press the switch in to the off position where it says off.
This switch below would be in the off position:
You want to make sure to read the switch, because I have seen some of these switches with the on/off reversed, which the pic below also has the switch in the off position:
Now, the fact the previous owner had the water heater unplugged could point to previous problems................or a safety routine some people practice.
Usually, with the Suburban water heaters, you only turn on the electric heating element, by using the black switch you posted a pic of on the outside of the water heater, which is similar to the two above.
I said usually, because as with the Atwood brand heaters, the Suburbans can be wired with an Inside the RV switch (or secondary switch) that can be used to control the electric heating element. When a secondary switch is being used, the outside black one (or primary switch) is always left in the on position..................and the secondary switch is used to control the electric current going to the heater.
The reason people like the secondary switch, is it allows them to turn on the electric heating element without having to go outside and flip the switch on/off.
Now, here is where it may get tricky for you as the new owner trying to troubleshoot. The electric heating element secondary switch can be a factory installed item, or something that the previous owner decided to do for himself. You need to look around to see if this has happened, because if there is a secondary switch somewhere, you'll drive yourself crazy trying to trace down why you aren't getting electric power to the heater during your testing/troubleshooting of this.
Usually these secondary switches aren't factory installed on Suburbans, but if so, they are usually located inside where the water heaters gas/DSI switch is. If customer installed, it can be anywhere, including inline in a way that it actually controls the electric outlet your water heater plugs into......instead of inline with the water heaters electric element. Either way, the outside switch has to be left in the on position.
Some people also leave the outside switch in the on position and just use the circuit breaker inside the RV that controls the outlet/circuit the water heater is using to turn the heating element on/off.
I have a feeling that the previous owner though, may have had a problem with the electric element. Either it's already been burned up, or he could have had something else go wrong (ie thermostat, on/off switch)........which is why he had it unplugged to keep from tripping circuit breakers..
Also, If the on/off switch went bad (stuck in the on position), then he would have had to unplug the water heater from the outlet, or turn off the inside breaker switch to it in order to not burn up the electric heating element.
I mentioned a safety routine earlier on, as some people unplug the water heater during winterization to make sure they can't accidently energize the electric heating element...with the bypass valve open, that would burn up the element. They plug it back in when they unwinterize the RV and close the bypass. It hard to know why the previous owner had it unplugged really. Was it because of a problem, or a safety routine?
The fact you are tripping a house breaker would seem to indicate the either the electric heating element is burned out, or the electric hi-limit thermostat shorting....or both which can happen if turned on with no water in the tank.
We have several articles in the FAQ section of Forest River Forums that explain how to test/replace these items.
Good luck, and I hope other members offer better advice and solutions.