That manual certainly helps. I looked around and found a manual from Atwood, but it doesn't have all the information that the one you supplied does. That single switch on page 7 is what I have.
The power for the element has been taken from a near by outlet. This means that it is using the outlet fuse instead of its own circuit. This says to me that the power is on as soon as the RV is plugged into shore power. There is a place for another breaker on my inverter/converter, so I may look into connecting the heater to its own circuit. Again, this seems to all point to the heater being a replacement.
I just found my multimeter, so I should now be able to figure out the switches.