If you really want a green skoolie do the following:
1. Pull the diesel engine and sell it.
2. Install a large electric motor.
3. Install LOTS of batteries.
4. Cover the roof with solar panels to help charge the batteries.
5. Drive 20 miles until the batteries drain down, recharge and drive again.
Buses were never designed to be "eco-friendly", unless you consider installing a composting toilet being green.