I think as it stands all the routes you want to use are open (assuming by Cairngorms you mean A9). However a week is a long time in Scottish weather, and I'd be keeping a general eye on Traffic Scotland > Traffic Info
Generally speaking it is accurate for any major issues (even on small roads).
The route you suggest is mostly "main" roads for Scotland and so even if they do get closed by snow you would NORMALLY expect them to be cleared and open within 12-24 hrs. Obviously if there is really bad snow or drifting then that might not be possible.
Whilst on a weatherman's scale its not that cold - the wind chill / damp etc mean is it bitterly cold in the last week or two and I wouldn't fancy sleeping in a van unless it was well equipped / insulated / heated etc.