Keep it!
My Bluebird has a metal headliner with 1 3/4 inches of fiberglass insulation behind. What about leaving it in place and adding 3" of spray foam on top?
I've got the same thing and I bought my bus with a minimal older complete conversion. When I say minimal, all of the amenities of home, small kitchen, bath, full electric, etc, but no insulation on the floor (peal and stick carpet on original floor) and original roof ceiling panels and insulation.
I am currently here at the Awesome Possum DIY ranch in Texas where it was 103 today. Granted I ditched the old roof top AC and installed a 9000btu minisplit, the roof is cool to the touch - even with the old AC.
I don't know why people remove the panels. They are part of the structural integrity. I did some research on this and by removing the panels, you add to the chance of a more disastrous outcome to a collision or wreck...by way of weakening a system that is part of the designed safety perimeters for protecting the passengers.
I've lived with the metal ceiling for over a year now. I have been slowly remodeling my bus, and in the near future I am going to have a wood ceiling to go along with my wood floors (mounted to original ceiling) I don't feel the need to add more insulation. Even with the little insulation I have in this bus, it stays cool, and even at these temperatures.
One thing that I did learn about sub-zero temps this last winter is...the floor definitely has to be insulated. It's like a freezer when it gets cold. Though instead of adding additional height to the floor and needing a roof raise, I plan to spray foam under the bus.
The moral of the story, keep the panels!
~Robert
"Old School"
1981 GMC 11 Window Blue Bird