My mother's very small bathroom has one wall she could use to put a small, 3 shelf unit (weight 10 pounds). But when we went to install it, we discovered that there is a sheet metal vent approximately 1" in back of the dry wall which is some kind of vent for the apartment building. So no anchors work. Is there anything anyone else can think of that we can mount the shelf with? The toilet is on that wall and there is virtually NO horizontal space on either side ... otherwise we would have done one of those shelving units that stand around the toilet.
You might be able to use an "EZ Anchor" (that is a brand name but there are other makers). They are only about 1 1/4" long and you can even shorten them by cutting off the tip if needed, to accommodate for restricted clearance behind the wall. They hold plenty of weight for lightweight shelving and knick-knack type stuff (I would not use them to support a TV).
So... 1/2" for the drywall, a bit for the thickness of the shelving unit, plus an inch of space between wall and vent, they should work perfectly and you can get them at almost any hardware store. Please shop at your local mom-and-pop store and buy American made products when possible! :)