Two bits of advice regarding the gravel road Alexander Bay to Sendelingsdrift:
1. If it rains on the bit closer to Alex, the road surface becomes very slippery and muddy. If it rains a lot it is impassable. Otherwise it is a very easy road.
2. Take extreme caution for mine vehicles - especially closer to the Park. I had a very close call with a mine pick-up towing a generator over a blind hill crest - at high speed in the middle of the road.
As far as the Park itself goes:
I visited it quite frequent up to 2010 and in that time noticed a few things.
1. The condition of the roads vary a lot. Flash floods wash the roads away at some places and get repaired by Parks staff, travelers as time goes by. Depending where you hit this cycle the roads can be passable with a 4x2 or not.
2. The bits that does not get subjected to frequent wash away of the road surface are extremely corrugated - to the extend that it is very unpleasant to drive on - in my opinion way worse than Kgalagadi. As a 4x4er I love a challenging road but corrugations does not fall in that category. I've seen more than one trailer left on the side of the road - shaken to pieces by the corrugations.
From my last experience you should be fine to visit Potjespram but take care as some (optional) bits close to and in the camping area are very sandy and will bog you down. The Halfmens Pass was easy going so you should also be able to drive to the Hand of God (S28 07 18.8 E17 00 34.1) but that is as far as I would go with a 4x2. Akkedis Pass and Helskloof Pass may be a bit much for the buss - if you consider the potential problems you can hit there it is not worth it really.
The RTP is a very special place. Enjoy