A studfinder (handy household tool available at HD, Lowes, or other big box) should tell you if you have something solid. Most RV outside walls are no more than 1.5" or 2" thick. You should be able to judge wall thickness at a door, window, or outside compartment opening. Depending on what you find, I'd think about limiting screw length to no greater than 1.25"
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter