Man, I'm not understanding what it is you want to do. What you have indicated in your picture seems to be the ceiling panels, which are screwed into and span the distance between the roof ribs. What is it that you want to fill in?
Some folk have taken down those panels, removed the possibly moldy fiberglas insulation behind them, and put new insulation back in. That might consist of flexible sheets of insulation, rigid sheets of insulation with slits cut into them to make them flexible, or spray foam (the best and most expensive solution).
Then, they either put the metal sheets back up (difficult) or figure out some way to put other ceiling treatment up, usually some form of wood strips.
Tell us what you're trying to accomplish and we'll help you get there. Oh, and welcome to the group! Oh, and please go to the "User CP" at the top left and fill in some of the details about yourself and your bus.