Locating aluminum frame in side wall?

Does anyone have a good method to find the aluminum frame in the side walls?  I tried a battery powered stud finder with no luck at all.  A factory tour video I watched on this forum shows the general location of the aluminum channel and another video showed the outline of them with heavy morning dew on the outside of the side walls.  Just trying to find a fool proof way to locate it if I want to mount something securely.

Goldenpod, we had the same concern with toilet access, as well as when exiting the Pod.  We got 12" grab handles from Lowe's and mounted them into the frames of the toilet (likely wood) and the entry door (probably aluminum).  They have been in place almost a year and they work very well.  Before we added the double step, that handle was a BIG help dropping down to the old single step.  The one at that door is best when exiting, while the big outside grab handle is more useful for getting in. 

I don't think you will have any problems with them pulling loose using the screws that are supplied with the handles.  One thing to remember is to use plenty of silicone around the handle in the john, as it will probably get exposed to water.