[JRMichler]

That series hybrid will need one huge electric motor. Just to throw out some numbers:

Assume tires 40" diameter.
Assume 80,000 lbs gross vehicle weight.
Assume it needs to pull up a 10% grade at low speed.
Assume geared for 1260 RPM at 55 MPH, that would be a 2.5:1 rear axle ratio.

You would need 8000 lbs traction force plus another 1000 lbs for friction and whatnot.
That's 9000 X 20 / 12 = 15,000 foot-lbs torque at the drive wheels.
15,000 / 2.5 = 6,000 ft-lbs at the drive motor.

If the main engine is 450 hp, that would feed a 400 hp electric motor.
The electric motor would be rated for 6,000 ft-lbs up to 350 RPM (400 hp).
Above that, it would run in constant power mode.

I'm not aware of any off the shelf electric motors anywhere close to those specs. Which would explain why the big manufacturers did not develop series hybrids for Class 8 trucks. In order to get the motor frame size down to where it will fit in the truck, I think it will need liquid cooling.