Ruska is the autumn colour, and in a country that is largely forest it is a season rather than a detail. It starts in the far north in early September and moves south over three or four weeks, which means that if you are mobile you can follow it.
Timing. Northern Lapland peaks around the first two weeks of September, Rovaniemi and Kuusamo a week or so later, and southern Finland into early October. It varies by a week either way with the weather, and Finnish forecasters publish ruska maps through the season.
Why it suits a camper. The mosquitoes are gone. Campsites are open but quiet, and you can generally arrive without booking. The roads north are empty compared with July. And because the colour moves, having your accommodation attached to the car is genuinely the right tool.
Where. Kuusamo and Oulanka, Pallas–Yllästunturi, and Urho Kekkonen National Park are the classics, and the fells make the colour visible in a way that lowland forest does not. Further south, Koli over Lake Pielinen is worth the detour.
What to expect from the weather. Days of 5–12 °C, cold nights, and a real chance of the first snow in the north by late September. Heating will run. This is a trip for a camper that is properly winterised rather than a summer van.
One practical note: daylight shortens quickly at that latitude in September — you lose about seven minutes a day. Plan shorter driving days than you would in midsummer, and you will spend more of the trip actually looking at it.