Let me take a stab here. At one point, I had to remove the exterior molding that covers the join between the sidewalls and the front wall of my TM. Once the molding was off, I could see the construction of the sidewall and front wall, as well as the way they were attached to each other.
There was no wood anywhere that I could see. As I recall, the front wall was framed with 5 pieces of aluminum channel - across the top, across the bottom, up and down the two sides (these pieces were notched and bent to form the curve in the front wall), plus one full-width cross piece behind the bend above the front window.
Each sidewall had a bent vertical piece at its front edge - I believe also notched and bent, to conform to the curve in the front wall. The joins were rather crude - the wall panels were simply overlapped and through-bolted at the overlap.
A couple thoughts. First, there is plenty of aluminum framing in that area. When you poked a drill bit into one of the suspected mounting holes, and it went straight through with no resistance, perhaps you were poking it through the existing hole in the aluminum channel, a hole that was intended to accept a relatively big lag bolt. The drill bit would encounter no resistance as you pushed it through, and would feel like it was simply passing through the foam, all the way to the outer skin. But the threads of a lag screw would bite into the aluminum.
Second, in my TMs, if I opened the cabinet doors, I could see that the interior skin at the bend in the front wall was not fastened down in any way. It was simply bent to conform to the curve, but then left hanging. Sloppy build, I always thought. By any chance, is that skin also loose in your TM? If so, you could pull it out a bit and see what is behind it. Or is build quality has improved and the inside skin skin pieces have been fastened down in some way, perhaps you could unfasten them and see what is behind them.
Finally, if the situation continues, you might consider removing that external molding at the curve, to get some idea of how the front and side walls are framed.
What year is your 3124? The transition from wood wall framing to aluminum framing happened gradually, in the 2001-2003 time frame.
Bill
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