I purchased an inexpensive
hotel-style towel shelf at Lowes for installation in the bathroom of our fifth wheel. It's the only one that I could find that fit perfectly in the space above the toilet.
As I'm sure many can relate, there are few, oddly placed studs in the walls and only thin paneling. Instead of using wall anchors, I decided on a more robust solution not likely to jar loose. I hand finished a piece of hardwood and fastened it to the wall using two countersunk dry-wall screws. The towel rack is fastened to the header using stainless steel screws.
Overall, I'm pretty pleased with the installation.