northern lights.

Wrap up warmly. I'm keen to see the Northern Lights but not that keen.

IMO you would be better flying there, Norwegian have some very reasonably priced flights.

I would think you would need to go at least as far north as Tromso for reliable sightings.  If you want to drive, you will need winter tyres, and I would suggest driving via Sweden.  The roads are much better, the fuel is cheaper, and there are no road tolls, in Norway there are far too many road tolls. 

I doubt there will be many, if any, sites open, other than ones near ski centres, but you can easily wild camp, and there are MH fill and dump points along the way, which may be open.

To get to Sweden you will need to drive either via Denmark and the bridge, or across Germany and the ferry from Sassnitz to Trelleborg.

Is your MH fully winterised?  At that time of year you could find temperatures as low as -20 to -30 degrees C.

You would most likely  need refillable gas bottles as you will use a lot of gas to keep warm.