W900 that's way too slow.

Neither nose cone or even fairings, not my trailer, though I ran an open flat with it for a bit and that helped (.3 mpg) but not my trailer

 

Difference is closer to 12 cpm on about 80 k miles a year so 9600$/year but who's counting?

 

Even though the w9 has a negligible trailer gap and the stretched t600 (1994) has close to a 9 foot gap?

It's possible, but Im quite skeptical of that

 

You're missing the point, my t600 has enough room to park a car between the trailer and the back of the cab, the w9 doesn't, by that alone, you lose the advantage of the t600 aerodynamics

 

Any chance I can ask what your preferred speed was?

Your pete sounds pretty dead on like what's in my t600 (which I like) I generally settle into 68 mph at roughly 1300 but it's longer wheelbase offsets the mpg savings it otherwise has on tall loads (my t600 tries never to pull a box)
Main difference I see is I have short rubber 24.5s