Shannon,
As Mark mentioned, I too elected to not use the factory supplied holders.
We went to Bed, Bath and Beyond and purchased Moen brand towel bars (2 ea at 18" in length), a toilet paper roll holder and a robe hook.
Yours is tougher in that your bathroom door slides. I mounted 1 towel bar inside the commode room door and the other on the wall behind the commode up high. I mounted the toilet paper roll holder on the outside wall (left side of our commode as you sit on it) and toward the corner of the wall in front of the commode. Then in a space on the exterior wall between the medicine cabinet and the shower stall, I mounted the robe hook. We use the robe hook to place our towel before we shower.
Moen product can be bought at most of the big box home improvement stores. It is not inexpensive but we wanted the nickel finish that matched everything else and the hardware that comes with it is great.
Each side of each towel bar and the toilet paper roll holder comes with this huge aggressive threaded aluminum screw. It would be great for a sheet rock wall but I found them to work well too for the hollow core door and the exterior walls of the trailer. Lots of bite and holding power. I also used a 1" #8 coarse threaded deck type screw in a hole opposite the big fasteners.
Best of luck with your placement and mounting. Do it once - do it right as those big holes will be hard to hide