non-invasive ceiling insulation
Like you, I chose to keep my bus ceiling intact, with its original fiberglass insulation. Then I added another layer or 3 of my own insulation to that.
If your bus ceiling happens to be the same size as mine, 8 feet wide if the arch were laid out flat, with a metal rib running from front to the back of the bus on each side, then as soon as the election is over see if you can score any 4 by 8 foot plasticor political signs. They resemble corrugated cardboard, but made of plastic. They wedge perfectly between those two ribs along the sides, and to cover your ceiling. I have two layers of that reflectix foil bubble foil insulation behind these plasticor signs. If you paint the signs with a gripper primer, then you can use any paint you want on top of that. Or in my front room I used automotive headliner adhesive to attach a really beautiful fabric mural to the plasticor signs. Then a few of Home Depot's cheapest fake wood trim strips to hold it up about every 4-6 ft where it wanted to sag with the weight of the insulation. At first I was using aluminum trim strips for that, which were more expensive, so I spent too much before finding the faux wood trim.
And whatever you do, don't drill any holes or screws into those two metal channels just above your windows that run from front to back of the bus. All of your wiring to the back, tail lights and brake lights and everything, runs through those channels. I was really young and alone and didn't know anything when I bought my bus 22 years ago, so I hung things from long screws drilled into those channels, then suddenly had no working lights on the back of my bus.
Good for you for charting your own path, finding your own strength and wisdom, and building a life of peace and joy! You'll find good people along your path.