i too would recommend power washing, but in with some or the oter cleaners, i would recommend Murphy's Oil Soap. its especially made for wooden surfaces. you can find this product at Lowe's, Home Depot, etc, even the local "big name supermarkets" in your area.
now, if there continues to be an odor of oil, which even after cleaning, it can still smell, i would recommend an old (very old) LTL trick, and that is, buy up to 3 pounds of coffee, any coffee, even the "instant" coffee..........
then using one pound, spread it all around the trailer where the floor meets the walls, in that it will be away from the main floor, and it should work its way into the cracks...........the smell of the coffee will in time, get rid of the oil smell..........you would rather have your customers smell coffee, than oil...
you might need to re-apply the coffee just before you go to a customer, then you can always say to the customer, damn, ever since that load of coffee got damaged by the fork lift guy, this trailer makes me want to stop at every coffeee shop from here to..........??
this is of course, if the customer says anything about the coffee smell, and i guarantee you this, that customer will never know the old LTL trick...........