On later models there is typically an inside switch for the WH on the control panel as well as the switch outside on the WH itself. But earlier models often did not have the inside switch, including my 2007. My circuit breaker panel has a breaker labeled WH. Use your meter to verify you have 120 vac on the load side of all the breakers..
The little black switch on the WH has a history of failing. It could also be a bad thermostat/hi temp cutout. With the switch on and using your meter start at the heating element and work backward toward the switch to see where power ends. If no power to be found, you may have to pull the switch to see if there is power to it
When doing the above, put the meter ground lead on a good neutral, or on frame ground. Reason is that there have been some who had a WH problem and it was found to be the neutral wire connection in the junction box on the WH had a loose connection causing the problem.
You can find a wiring diagram of the WH by googling Suburban RV water heater manual for your WH model.
Bill & Patricia
2012 F350 SRW CC LB FX4 6.7