temperature dropping to just below freezing as the dirty-white tour bus trundled

It was cold that rotten dark morning, the temperature dropping to just below freezing as the dirty-white tour bus trundled along the old single-lane
highway. Still only late September but in Sweden, where in summer the sun never sleeps, the nights were now growing long again. Mick Wall "Enter Night"
Does the bolded structure imply that the action of "dropping" was happening simultaneously with "trundled"? Or had the temeperature already dropped by the time the bus tundled?

The temperature was falling. It was getting colder. At that same time they were moving along that single lane highway.

While the bus was moving, the temperature dropped from above freezing to just below freezing. The drop might have been sudden or gradual. There's no way to know.

There are no implications. It is stated plainly that it was getting colder, and the bus was moving. Those things were happening at the same time.