The medicine cabinet in our RV was generously sized (in our opinion), but it only had one shelf. There was a lot of wasted space.
While we could have stored about 16 super-tall cans of 80’s Aqua Net, we’ve both outgrown that phase and our toiletries are a bit smaller these days.
The cabinet itself is made of a light ¼ inch plywood, so it is not a good base with which to attach a shelf. However, I did this, I wanted to avoid visible screws. I chose some light hardwood lumber that could be stained to closely match the rest of the cabinet. Rather than installing standard shelf supports (which would likely require visible screws), I made two legs and used double faced tape to attach them to the inside of the lower shelf in order to support my new shelf.
I cut and stained the new shelf to match as well.
The one additional shelf added a lot of room in the cabinet. If we find ourselves running out of space again, I could add an additional shelf to the lower area in the same manner. (Or, more likely, we do a little spring cleaning.)