My manual is a bit newer, but here's what you have according to the model number chart. It is a Atwood combination gas and electric heater with a 6 gallon capacity and pilot ignition only.
That means there is no electric circuitry on the outside of the heater behind the drop-down white door. It is strictly a manual pilot light heater when on propane.
To use it on 110 vac electric, fill the system with water including the water heater. Then plug in your shore line to an adequate 110vac source (at least 15 amps.) Locate the circuit breakers and make sure the one for the water heater is on. Then turn on the switch on the top/back of the heater. If the element, thermostat, and the high temperature limit switch are all ok, and the wiring is good, and no one added a hidden extra switch in trailer somewhere, you should soon have hot water.
If no hot water, remove the translucent shield and test the wiring with a multimeter or ac probe. If all the electricity is present at the element, then the element is suspect and would need to be replaced.
If there is no voltage at the element, then check out the rest of the 110 vac system. Trace it from the circuit breaker to the water heater. We had another user here who had the same problem on a much newer trailer. Turned out that a previous owner had installed a switch in the line to turn the heater on and off from inside the trailer.
It is possible that either the thermostat or limit switch is bad. You can by-pass them one at a time to check, but do not operate the heater for any appreciable length of time with either by-passed. The results could be catastrophic.
On 110vac, all the circuitry is shown in this picture:
The little black switch on the panel is the on/off switch. All of the wiring is visible in the picture.
The long narrow translucent shield covers the heating element and the thermostat. Behind the shield on the left is the heating element. It is a miniature version of the heating elements used in home electric water heaters. In the center behind the small opening in the shield is the manual reset high temperature limit switch. To the right behind the shield is the thermostat.
This schematic should be correct for your unit:
Before working on it, trace the schematic to your heater and verify that it is correct. If it is not, you may want to contact Atwood for a schematic just to be sure. My schematic is from the version 7 of the heater. You have version 6 which should not be much different.
The heating element is removed by disconnecting the wires and using the proper size socket. Be careful with electric heating elements as they can easily break if you torque them too much when installing them. You really have to use a socket on them. Any other wrench or pliers will break the new one on installation. Discard the old gasket and use the new one that comes with the element.
Remember to completely disconnect the electric to the trailer before working on this.
A replacement element can be purchased at any good RV parts counter plus there are several online sources.