I don't know if they would still do this, but a few years ago I got someone at FWC to send me pictures of the frame, taken at the factory of a unit being built. By measuring on the camper and scaling on the photo, I have been able to pretty accurately determine where the structural parts of the frame are.
There is a wide channel that runs along both sides and across the front (all one piece, curved around the front). You can see how wide it is from the outside by the height of the cabover part. It is thin aluminum, but you can cut threads into it for bolting things on. That's what I did for mounting my awning, but I used screws long enough to go all the way through the wall, with appropriate spacers and nuts on the inside, so the nuts tighten against the frame channel.
Alternatively, you could make a larger hole in the interior panelling, cut through the insulation and put a nut back in there, against the frame member. Then put some kind of decortive plug in the hole on the inside.
I don't think mounting holes that don't go through a frame member are a very good idea. The aluminum siding is very thin and soft and tightening anything against it would just deform it. Maybe you could attach something really light with sheet metal screws into one of the raised areas of the siding. In a location where there is a cabinet on the inside, you might be able to bolt through that, but you would need a spacer within the wall, to avoid bending in the siding.
The siding is not flat. By a frame member, the low parts of the siding are against it but the high sections are out about 3/16". For a bolt in one of those areas, you need to drill a larger hole in the siding and put a 3/16" long spacer (or some washers) on the bolt, to avoid crushing the siding when you tighten the bolt. You can get spacers like that from McMaster-Carr. I used a wood-cutting Forstner bit to cut holes in just the siding, for the spacers. The aluminum is really soft and it didn't seem to hurt the bit.
Use lots of caulk! Good luck...
- Bernard