No shortage of filling stations on your route , even between perth and inverness however be prepared to pay extortionate fuel prices up in the highlands . i was up past ullapool 2 weeks ago and diesel was 111.9 per litre . if i was you go to tesco's in inverness it was 102.9 there and plenty of room in the forecourt for car and caravan
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If you're coming up the M74 you could come off at junction 8 Canderside Toll, down through Larkhall to large Shell garage, and back on at the bottom end of Larkhall junction 7.
Also another good place to stopover is Lettershaws, just a couple of miles off the motorway at Abington.
http://www.plenderleith.com/lettershaws/ (strange, how do you do a link on this site, thought you'd just be able to click it!)
I don't bother too much about the price of fuel en-route, even 10p per litre difference only equates to a few pounds per tank and this is easily outweighed in my opinion by the lack of stress finding station off route. Having said that I always set off with a full tank.