As Chad mentioned, with respect to the controls an E12S is in essence an E15 without field weakening. If you haven't already, I'd strongly advise you download a copy of the Homeowner's Service Manual: http://www.myelec-traks.com/OPS-74-4035.pdf
It details the differences between the two models, and has all the appropriate troubleshooting diagrams and data.
Assuming it is functional, what does your "power use" meter indicate when the "fuel level" meter drops? Also, just a point of clarification (because it actually does matter) when you're measuring voltage, are you measuring across the two motor leads (or field leads), or from the motor leads to battery ground?
Check the motor armature and field for continuity to each other and for continuity to the case. Everything should be isolated. This is a good step to help rule out a bad motor.