I have a very similar situation. I have a truss roof placed directly
on top of a short kneewall on top of a single wide trailer. The
trailer roof is insulated with R11. The 'attic' is vented by a
continuous ridge vent with perforated soffits. I have experienced no
problems with this setup. Due to rising fuel costs this past winter, I
added R38 insulation directly on top of the old trailer roof, between
the studs.This was quite effective, but expensive (this is a 65
footer). I also insulated between the rafters with R21. Unnecessary I
know, but I have found the heat buildup in the 'attic' was excessive,
and I wanted to stop it at the source (My roof has a Southwest
exposure). I used something called 'rafter-mate' , a foam channel that
directs the soffit air up and out the ridge vent. So far this has
worked very well.
this site has a lot of metal roffing info.