TM 9-2350-247-10 includes a 2-page list of additional “kits” that can be added to the M548 (pages 1-2 through 1-4). There is no mention of a steel roof. However, there is a Cab Secondary Personnel Heater kit for use when the temps dip below -25F which includes an insulated fiberglass cover to replace the canvas one. There is similar secondary heater kit for the cargo area which includes an insulated fabric cargo cover.
I took my M110A2 battery and M548s out the field once when the temps hit -10F. No secondary heater kits on either vehicle. We froze our asses off. I’m lucky I didn’t get fragged…
My thoughts were similar to yours as far as the open bed, fuel hauler or maintenance type vehicles. That’s why all those hold downs and brackets in Rick’s pictures would stand out. Wayne
TBH though I don’t think too much of that detail would be visible with the refueling gear in place, though if you’re going for a general cargo load or 100% accuracy maybe there’s a 3D decal set out there.