most of us improvised...like scrapping the toilet paper holder and buying one that free stands on the floor and is heavy enough to stay put...ours fits in the garbage can that sits in the shower while traveling. For a towel bar, many also buy a towel holder that can fit on the door to the bathroom, the ones that hook over the top of the door. Most don't want to drill holes into the thin walls.
Some have mounted TP holder inside the vanity door. There's not a lot of options in most trailer bathrooms.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.