Typically the walls total thickness is about 2" When Installing things on any walls in my rig I use nothing longer then 1.25". Something to consider is that in many areas it is best to install a doubler to support what you are hanging.
Jake my sidekick (yellow Lab)
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho