Do you have a "wet" shower, where the toilet is in the shower area? We do, and the black tank has a vertical vent pipe in the corner of the shower enclosure -- I used cable ties to secure a small plastic basket to that pipe to serve as a shower caddy. Be sure to drill holes in the bottom of the basket -- otherwise, mold and mildew might grow in the bottom. Don't ask me how I know this.
If you don't have a pipe or any other place to secure a basket for your shower items, you may have to put a removable basket on the floor of the shower and then stow it away when you are on the highway.