I renovated the bathroom in our Class C back in 2016. I installed new wallpaper on a section of the (new) hardboard wall, behind the sink/vanity and toilet.
The wallpaper I selected was designed for bathrooms, moisture resistant and washable (available on-line from Lowes). Graham & Brown makes kitchen/bathroom wallpaper under the "Contour" brand.
I discovered after our first wallpaper hanging attempt, and subsequently talking to the wallpaper manufacturer, that I needed to prime the (new) wall surface first. This is so the wallpaper glue wouldn't be sucked into the new (hard)board. Our second wallpaper hanging was successful. Wallpaper has stayed in place for 4 years now.