Anyone installing anything on/through the walls of a fibreglass sided motorhome, needs to know exactly where the steel frame is located. It is not a good move to drill through ANY frame member, as this can severely weaken the structure. Winnebago owners are in a bit of strife though as the only steel is used in the roll bars. The floor however is all steel frames, BUT look out for sheets of gal used to hold cupboards to the roof. This gadget works on magnetism, so it will only detect STEEL. It is used in the steel frame house industry - Newsteel etc. have them. All you do is hold it very close to the surface and watch the needle. When it is on top of metal the needle moves through 90 degrees and "quivers" (that could be us shaking with excitement though at finding something!). You can get more info by emailing CMCA member "Curly" Alcott - Swagman Signature. nalcott@tpg.com.au