The leading arm axle
is welded into the 90* corner made by the rear floor rising. The welds run along the horizontal frame AND vertically where the frame goes up for the "dance floor" of the dinette/bed.
To get a trailing arm you have to find a location in front of wheel well where arm will reach back and put the wheel in exactly the same place. Plus (important!) do something to replace the structural strength the welded bracket added to that 90 degree joint and axle
tube provided to the rear of the frame. One advantage of the torsion axle
over leaf spring is the torsion axle tube is a structural part of the frame.
Don't get me wrong, people do make this switch. There are threads by folks that have done it. It is just not a trivial change and you need someone that is good and will do careful work.
Airstream and Scamp
used leading arm, not sure why but I think leading arm and torsion axle was considered a superior suspension to leaf spring suspension typical of campers back then and still the more typical axle today.
At some point in time trailing arm became more common and now is the typical orientation of torsion axles. Dexter axle you just turn around and swap the brakes
to the opposite side to turn a trailing to a leading BUT your guy has to reverse the dimensions when ordering AND the bracket may need to be reversed or cut to fit.
I'll try to find a picture but the rear of the scamp
axle bracket is Flat to fit into that riser corner AND the bracket offset from the axle tube is a bit different. Projects further one way than the other and not sure it is the "common" direction.