How long are you planning to be on the road?
To state the obvious: it’s going to be cold, especially in a roof tent. Do you have any heating in your car?

The roads will be snow and ice covered. Get the best winter tyres for your vehicle and remember snow chains which are compulsory on some roads.

It’s going to be dark a lot if the time, this makes driving tricky especially in snowfall.
The weather can change very quickly, and what was a clear road in beautiful sunshine one day can become covered in feet of snow and with zero visibility the next morning. Take provisions

Most campsites will be closed, don’t plan to rely on them without checking websites or phoning. Most public toilets in lay-bys will also be shut in the winter.

Wild camping in vehicles is not really allowed in Norway, but usually tolerated off-season if you are discreet.

Public swimming pools are great for getting a “deep clean”, there’s nothing like a few showers, a swim and a sauna to make you feel sparkly again!

Have a great time!