Rovaniemi is about 830 km from Helsinki. Levi is roughly 1,000, Saariselkä about 1,100. In a car those are long days. In a camper, at 80–90 km/h and with stops, they are two days each way — and treating them as two days is the difference between arriving tired and arriving ready.
The route. Most people take the E75 north through Lahti, Jyväskylä and Oulu. It is the fastest and the best served: fuel, dump stations and food are never far. The coastal alternative through Vaasa and Kokkola is prettier and adds a couple of hours.
Where to break it. Roughly halfway puts you around Jyväskylä or Oulu, both of which have campsites open year-round and are worth an evening rather than just a car park. Kalajoki is a reasonable detour on the coastal route.
What the drive is actually like. Finnish main roads are quiet, well surfaced and straight for long stretches. The real hazards are reindeer north of Oulu — genuinely, especially at dusk — and elk further south. Both are worth slowing for; hitting one in a 3.5-tonne vehicle is a serious matter and you must report it.
In winter, add time rather than pressure. Roads are ploughed and salted or gritted, and studded winter tyres do most of the work, but daylight is short: in December you have perhaps four usable hours of light in the north. Plan to be parked before dark rather than pushing on.
The reward is that once you are north of the Arctic Circle, everything is close together, and you are carrying your accommodation with you in a part of the country where hotel rooms get expensive.