Do a search in the archives for "diesel smell" or "diesel odor" or "diesel cushion". Peggie Hall has provided a good writeup on how to do that.
Generally, the bilge needs to be cleaned thoroughly with water and (a little) soap and a scrub brush. Let dry and spray liberally with PureAyre - let dry. For the cushions, you can try removing them; spray completely again with PureAyre and let dry. That should work unless there was an actual spill on the cushions. If that is the case, you'll have to remove the covers and clean them as well as spraying the foam down with the stuff. PureAyre is safe for most everything and is not supposed to leave a stain. There is an Expert forum where Peggie Hall has a section called The Head Mistress and lots of info there on sanitation systems, odor treatments and the like.