I have put grab bars by the steps (and if possible across from the toilet) on three different fivers for my now 95 year old MIL. I have always found where I could go all the way through the wall(s) and put a fender washer and acorn nut on the other end.
The attaching bolt must be cut to just the right length for the acorn nut to take up firmly without crushing the hollow wall. Put a drop of Lock-Tite on the threads.
You can get washers and nuts in Chrome or paint them to match the wall covering with acrylic craft paints from Michael's on somesuch.