We decided to rent with Arctic Campers because it was one of the only companies we could find that would rent us a camper for fewer than 10 days (or that could accommodate 5 people!)
Granted, this means we needed to return the van in Tromsø, and couldn't do a one-way drop-off, but it allowed us to start our journey in the north, drive south toward Lofoten via Senja and Andøya, and do the trip as a loop.
When arranging to pick up your van in Tromsø, double check the opening hours, since the facility isn't always staffed, and expect to spend about 1-2 hours going through the orientation, learning about the van, and signing all the of the paperwork.
Technically, you could walk to the office from the airport (it would take about 45 minutes), but the easiest thing to do is to catch a taxi at the airport. It's a few bucks more, but it will give you more time to get out and hit the road!