Turns out my coach has the 2″ sump based on inspection – the actual external measure of bottom of pan to bottom of trans case is closer to 1 1/2 inches. Pan is easily visible looking in from either side of the coach. The nominal 4″ sump is actually about 3 1/2 inches from what I found in research. So, my unit takes the 4″ filter. The 6″ filter is for those units with a 4″ sump.
Also found no definitive statement of such but from parts lists and illustrations I encountered in researching this it looks like the determining factor of the deeper 4″ sump is if the Allison unit is equipped with a PTO. Doubt if many coaches have that, unless someone switched out a unit as a replacement at some time in the vehicles life. Or perhaps the OE or converter customer specifies the deeper sump when specing out the drivetrain. Contrary to what I would presume due to the extra fluid capacity of the deeper sump, the prescence or absence of a retarder did not appear to affect the sump depth used.