update (12 February 2019): Diesel is available again!
According to a posting in the SANParks - Kgalagadi Transfrontier Park facebook group by the Regional Communications Manager: Arid Parks at South African National Parks (SANParks), Genevieve Maasdorp, there is currently no diesel at Twee Rivieren Rest Camp in the Kgalagadi Transfrontier Park. Visitors using Diesel vehicles are advised to fill up at the town of Askham before entering the park.
The Twee Rivieren Rest Camp is situated on the banks of the Dry Nossob Riverbed. It has a reception, shop, fuel, restaurant, information centre and swimming pool. This is the only camp in the Kgalagadi Transfrontier Park with 24 hrs electricity and cell phone reception. The town of Askham is located about 200 km north of Upington at the junction of the R31 and the R360 roads.