I did Cape Town to JHB on a regular basis. What I have done is to drive as far as George / Knysna and spend about two nights to ease the journey. From there, on to Port Elizabeth and possibly Addo Elephant Park. From there, onward to Cradock (not sure if the roadworks have been completed), then spend the night in Colesburg. (It will be very, very cold this time of year).

From there it is a decent run to Johannesburg where you can stay for a night or two to relax, then go to the Kruger. For accommodation options en route, get the latest copy of "Country Life". This is especially for Knysna / Colesburg - some quaint B&B's advertise and make for interesting stays.
Also, look on Groupon for accommodation - we have picked up some interesting options over the years. It is school holidays though, so options may be limited, but have a look, you never know what will crop up.

Have fun, only drive during the day, and enjoy your visit.