The shower trucks are all slightly different in their layout, They all have some kind of shelf or rack to hold shampoo, soap, etc while in the shower. If they don't have a hook inside the shower, there will be a hook just outside the shower cubicle for your towel. These hooks aren't very big so you are better off hanging the majority of you clothes on the hooks above the benches.
The cubicles are pretty small and are are not designed as a changing room, it is a pain in fact if people get undressed, shower, dry and dress in the cubicle as it doesn't free up the shower for others. The usual drill is to get undressed near the benches, then shower - yell out - "shower free" for those cueing outside, then dry and dress near the benches. This speeds up the whole process. There is nothing worse than a shower cubicle hog! It's also really important to conserve water, so make the showers as short as possible.
On of the nicest things about getting back home after the GVBR, is having a nice long hot shower in private.