We were going to do the NC 500, but decided against it due to the size of our motorhome.
Instead we stumbled across the Motorhome and caravan magazine, highlighting what they call the SC 500. Starting out at Gretna Green, going round via Newton Stewart, Wigtown(Scotlands book town), Garliestone ( where they trialed the Mulberry Bridge for the D D landings), Whithorn, Largs and others. We went off piste a little, and went across to Dunoon, then round to Inveraray, Lochgilphead, on to Oban, round via Glen Coe. Superb roads, and still touring. Found some little sites on forestry land, where we wildcamped, ( only 2 motorhomes allowed @ £7/night, but must park 4 metres apart.) Jubilee Point on Lock Eck. Free camped around Largs down by the beach for a night. Still enjoying the countryside. Weather has turned a little, making our way back down via Annan water, Moffat and then back to a little motorhome aire at Gretna Green, where we started. Then home to Yorkshire.
Hope this gives you another idea/ route to try
Lots of history on this circular route, and allowed me to do a lot of fly fishing as well.