We used to live in an RV. The bathroom was beyond tiny. It was approximately 3 ft X 5 ft with a tub, leaving about 18 in x 26 in to stand in front of the vanity. Two people cannot stand in this “room” at the same time.
Without a doubt, the most challenging aspect of living on an RV is making this bathroom work for three people. Some of the things we did to make more storage was hang an over-the-shower basket on the wall with command hooks by the toilet. Here we keep lotion, cotton swabs and balls, and my flat iron. From the bottom basket we hung a hair dryer hook that was meant to go on the vanity door, but didn’t fit there, and store our travel-sized hair dryer. (though somewhat precariously placed above the toilet!)
We hung three command hooks on the back wall above the toilet for our towels, and we have an over-the-door metal rack to hang other things like my make up bag and hair items. The space under the vanity has some decent space. Beyond that, we really just pared down A LOT. And sometimes that’s the best you can do!