I don't know where you are looking, but why not go to a Bed-Bath-and-Beyond and see what they look like. I think WallMart carries some of these also. And places like Home Depot or Lowe's may have them too.
We have one in one of our bathrooms at home. I would think anything you can use in your home would work OK in an RV shower. However, as ours is in the guest shower, after some time of not using it, the soap gets gummed and when it finally breaks free, it fires like a cannon ball out of a cannon! Also, if it does not get used, I've noticed the soap and cream rinse and body wash begins to turn color and look nasty.
One thing that may happen by putting one of these in an RV shower, I would think the bouncing down the road would cause the liquids to squirt out, making a mess on your shower floor. I could be wrong.
Also, once they are attached to he wall, they are hard to remove. Although installed with that extra strong double sidded sticky tape, if you do want to remove it, that tape is just impossible to remove. If you use screws, you will forever have a hole in your wall.
Then you have to deal with the container in the winter months, which will probably mean taking it off the wall and bring it inside so the residue will not freeze and bust the squirter mechanism. I see you are from RI, you have cold winters there too to contend with and winterizing your RV.
Personally, I would never put one in my RV shower.