Similar to what was said about the concrete curing blankets/tarps I saw some insulated tarps on sale at a store up here. You might find some at harbor freight down there or on Amazon. Throw them over the roof and tie rope around the bus and I bet they’d make a pretty big difference as long as they’re thick enough. Remember heat rises so the roof is where insulation makes the most difference.
This link is to a popular store in Canada but they’d probably ship down to you and there are different sizes, this is the biggest. It’s priced in Canadian dollars so with current exchange it’s roughly 30% less in USDollars.