I recently went on a 5000km + road trip courtesy of GM. When you're burning that much juice it's a mercy to discover that there are some places that sell diesel cheaper than retail prices. I found one for you. It's also situated in one of the most convenient watering holes, slap bang in the centre of the country. That's right, just outside Bloemfontein. So make sure your tank is virtually dry when you pass next to that Caltex garage coming on the N1 from the Brandfort/Kroonstad side of Bloemfontein.
Instead of going left [coming from Kroonstad, ignore the Caltex and continue over the bridge. Immediately over the bridge on your left is a place selling diesel around 57 cents cheaper. I kid you not.