I think the tool that Haroon refers to is called a sink basket wrench also known as a spud nut wrench with two parallel finger-like jaws that adjust sideways. If you don't have access to one you can just as easily tap the bung loose, much like tapping the lid off a food jar except you're working around the inside edge. Use a drift (maybe 1/2" by 6" to 12" long) at a shallow angle into one of the bung notches. Strike the drift with a hammer a few times (counter-clockwise direction of course). Work your way back and forth on all the accessible notches. The bung will soon start to loosen. Penetrating oil could be applied first, but I've never needed it. You should be working in a well ventillated area and ideally you will use a brass drift (to avoid sparks), but an piece of bar stock or even a large slot screwdriver will do. There is plenty of room to work through the access plate hatch. Wagons have full headroom over the hatch and are a piece of cake. Sedans are only slightly more awkward. Just avoid slipping and skinning your knuckles or damaging a hose nipple. The hammer and drift method can also be used to re-tighten the bung.
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-Dave (not to be confused with a real expert, just goofing around at this)