I had good results with the Stugvik brand on the more rigid walls (attached to the outside shell and interior walls) on my 21 but the back wall (behind the toilet) didn't stick well with a 2-cup device like a towel holder. Finally played with how the wall acts during travel and found a whole lot more flex in it than the other walls. A single suction cup would stick fine, but a 2-cup device would lose suction on one cup after sitting backwards on the pot sipping a brew and deflecting it back and forth with a finger.
So I decided to use a shampoo bottle holder I use for Casita baths. Sticks out 3 inches, 3 inches high, 7 degree angled bottom for drainage, taped on with 3M RP45 tape good to 200F.
Wifey likes it!
Can an admin rotate this? I forgot iPhones have an attitude.
Need custom storage to your design? Don't drill holes!