Hello Gary and welcome. I too just went through that same process. I have a 3914.
Spoke directly with the factory by phone and was told the studs are on 16" centers but my stud finder does not seem to verify that. I'm using a Zircon stud finder (Lowes or Home Depot) that uses depth detection technology and it does find lots of "things" but not studs on 16" centers!
I used painter's masking tape, tore off a lot of small (about 1") pieces, and then started looking for anything that the Zircon indicated resembled studs. Everywhere I got a reading I put a piece of tape with a pencil mark to indicate the "center" of the reading.
Turns out there are horizontal braces, unique window bracing, some wooden beams (in the ceiling between the garage and the loft bedroom) etc. Even found some "studs" that were about 6 inches wide running vertically in some areas. I assume these are probably some plywood bracing pieces but I don't know.
Pretty regularly found aluminum studs running within an inch or so of all the door frames and window frames so when I decided to start drilling holes I made sure I hit at least a few aluminum studs and then added extra screws directly into the wall panels just to give additional shear strength.
If you start putting lots of tape up on the walls you will begin to see patterns of underlying support that you can tap into, and when you see recurring "solid" spots (running top to bottom) you can assume they are studs and you can then measure 16" across and "hope" to hit another stud.
And "lake texoma" said below to turn on your stud finder before you put it on the wall. The Zircons don't work that way. You must place the detector on the wall first, then activate the "on" switch so it can read the depth. It locates aluminum by determing the increased depth compared to the thin wall panel.
The metal setting does not detect aluminum, it detects magnetic steel or wires.
I also removed some screws from the "floor" of the loft bed area (over the garage) so I could lift an edge of the chip-board and peek underneath. There were definitely aluminum beams running on 16" centers from front to back, not across the garage. Once I located them I was very happy to go back underneath and confirm their existance on the ceiling of the garage by both using the Zircon and tapping to hear the sound difference.
It felt a lot better confirming that what I thought were studs actually were studs.
Hope this helps.