The water heater should have a junction box that the wire goes into. the box should have a removable cover, open it and you will see the wire connections.
The wire should run directly to the fuse/breaker panel/converter. it should not junction anywhere in the run.
The fuse/breaker panel/converter is probably going to be a brown box, located in a lower cabinet, with a drop down door panel that covers the breakers and fuses. This box also has a cooling fan. When enough of a 12volt load is placed on the system, the fan will kick on to help cool the converter. This should help you Id the box location (mine is located under the Ref).
With the camper unplugged, you can remove the screws to the box cover, and remove the screws that hold the box in place. On the back are ALL the electrical wires in the camper.
You need to figure out why the wire burnt. Replacing it is not a guarantee that the same thing won't happen again. It might have been caused by a short in the element, which can happen if the electric is left turned on when the water heater does not have water in it. There is usually a small (hard to see) switch behind the WH access door on the exterior. You may need to remove the element and check to make sure it is not toast.
Chuck Roder & Family
'07 Surveyor SV-235RS & '03 Chevy TrailBlazer EXT LT V8
High Springs, FL.