Stick to the most major roads you can and you’ll be fine. The only bit I usually advise against is the A82 alongside Loch Lomond but with a small van you’ll not be too concerned anywhere. A decent road atlas will show single track roads and gradients.
Unless you want to wander off the beaten track the roads in the areas you want to visit are OK. You'll find some narrower roads in places on Skye but most have passing places. Have a good time.
Andy and Liesl