If the contents of your holding tank spilled out and found its way under the shower tray area, then presumably there is a small gap between the inside of the hull, and the under side of the shower tray. There are various engine oil suction pumps around, and the one I have is similar to the one shown in the attached photo. It has a long thin tube that will fit down inside a dip stick tube on the engine, and possibly would be thin enough to be able to slide the end of it down through the gap referred to above. Once there you would then be able to extract the smelly stuff and dispose of it into your nearby toilet. Once emptied I'd suggest flooding the area under the shower tray with a mixture of fresh water and thin bleach, and leaving that in place for a day or two for it to do its magic. You may need to do this a couple of times, but bleach is pretty good at removing all manner of unpleasant stuff as well as the associated smell. Finally perhaps finish with one of the proprietary toilet cleaners which have something added that smells rather more respectable.
