I worked in Eribol, which is halfway between Tongue and Durness and it's not a very wide road and it is very twisty and up and down. Also if you are travelling from Lairg to Tongue then the road is 50 miles, single track with passing places.
Don't take the coast C-road from Ullapool up to Lochinver - it's single track, has blind bends with an unprotected drop to the beach on one side and a cliff on the other. We met cars coming in the opposite direction that couldn't reverse to the passing place, so we'd to back the MH round the corner. Hate to think how a caravan would fare. Fabulous scenery, but a scary road. OH is a bold driver, he says he won't do that road again in the MH. But you don't have to use it for Durness. You can see what anywhere looks like by checking www.geograph.org.uk which is an Ordnance Survey map site aiming for "a photograph for every grid square".
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We had a week here in June
Fabulous site, well worth the trip.
The road up past Kylesku is singletrack, but very good.
There are numerous passing places, don't be put off
It's well worth the trip
Enjoy
If getting to Sango is too much for you take a look at Clachtoll. Easy to get to from Lochinver, just a short distance on a good single track from Lochinver and also stunning place to visit.
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cwarchc, we have driven up past Kylesku to Scourie and don't remember any single track. We haven't gone as far as Durness, as requested by the original poster though.