I don't want to step on your toes or question your enviromental concerns but the actual amount of oil coating the the engine/gearbox etc wouldn't fill a coffee cup if you you could collect it all up. Good engine cleaners like Gunk emulsify the oil anyway. I read that simple green and Dawn dishwash detergent do a good job too. Paintbrush cleaner and brake cleaner work too. However. I'm in the UK where in the last couple of years hand car washes have sprung up around every corner manned by guys talking every east European language. They do a good job cheaply regardless of their legal status.
Maybe your local truck stop has a tank cleaning facility? Should be a 5 minute job for them, even with the pressure turned right down.