We had an OTJ (over the john) cabinet in our last camper. I found a stud to mount it to, and used toggle bolts for the areas where there was not a stud. No issues with it staying in place. We needed the storage, but it made a small bathroom feel even smaller. We elected to no hang one in our new current camper because of this.
With the ceiling and floor materials being thin, I would be cautious to not put to spring pressure on a floor to ceiling cabinet. You might end up putting dimples in the floor and/or ceiling. If it does not have enough spring pressure, it's not going to stay in place during travel.
Chuck Roder & Family
'07 Surveyor SV-235RS & '03 Chevy TrailBlazer EXT LT V8
High Springs, FL.