Before you get too carried away, there are a few simple diagnostics you can do to narrow down the problem. If the electric side worked before and you didn't accidentally drain the tank with the heater on, it might be fine. Those heater elements last a long time if you don't run them without water.
Far more common is the black rocker switch in the lower left corner. It could be tuned off or has just died. Pop it out and check for 120V.
The thermostat may have reached an over-temp situation. It is reset by pushing on the appropriate spot on the rubber cover. (The left side is the AC thermostat, right side is the 12V one). You should check it for 120V on both sides.
The water heater element can be tested by measuring the resistance (power must be off). It should be around 30 ohms and if open it is toast.
There are a couple of trouble shooting manuals in the Files section that will help find the issue:
https://www.montanaowners.com/forums...?do=file&id=57
https://www.montanaowners.com/forums...?do=file&id=56