With the exception of the Kruger National Park, not many safari parks or game reserves offer internal fuelling stations. If there are stations inside the park, they are usually far apart, which could lead to a half-full tank becoming an empty tank quite quickly.
You will need to make sure that your tank is full before you head off to spot lions, rhinoceroses, and elephants. When it comes to car hire in Cape Town, car rental companies will always fill the vehicles as much as possible, to where the fuel can physically be seen in the pipe, to avoid disputes about how “full” the tank is.