I use straight simple green, let it soak for a while, scrub it a little if you can get to it,and hose it out.  Did this yesterday, sprayed simple green on the in-bed fuel tank and between the tank and front of the bed because a lot of leaves had acumulated there since last year.  Then drove to a car wash that had a outside power sprayer, turned the soap on and hosed it all out.  Then a little plain water to finish it.  Now there's no more leaves and the diesel is gone.
Bill