Below was my original idea to build the walls. The studs on the inside and the ply wall on the outside. Pretty much how they build houses. Obviously the reason the external skin is put on first when building a house is because of tradition and weather.

Using this system all I can do is add wires and insulation before I attach the inside walls. Then I'll need to drill holes and fish out wires etc. The same process when fitting plasterboard in a new house.

I had one of those "middle of the night ideas."
I have almost no perforations in the outside wall (light switches etc) so why not switch the walls around. In essence in the photo above; the walls swap sides.
This means the plywood wall you can see is now the inside wall, not the outside. If the walls are erected, (with 6mm for the external skin) I can add switches, wires, cut slots for the roof spars. It also means I'm standing and working outside the trailer and not crouched inside it. Rather than erect it, I could lay it on the deck of the trailer. It allows me to put in additional timber, cut holes from the inside out and align switches/speaker.

Another though crossed my mind.

Thinking out loud...
It is possible to leave the outside wall off until the trailer is almost complete. This would mean the shelves, cupboards could be easily lined up with studs. More importantly if an additional stud is required, it's easy to add.
At what-ever stage, I can pack it with insulation, one or two holes for an outside light and then glue and screw on the outside walls