I’ve just been having a look at this issue and I see it comes in 2 parts:
1. The toilet, which is a “gravity fed flush toilet” for which there are several suppliers world wide but maybe not many in SA, and
2. The “black water holding tank” for which there are a lot less sources. If you haven’t seen it already try https://www.icondirect.com/waste-holding-tanks/. It seems like you just bolt the tank under the floor with the inlet located directly below the toilet outlet, and then there’s a handle you pull to discharge the tank, presumably into a large-diameter pipe that leads to a disposal site.
I think the big challenge will be to find a tank that will be the right size and shape to fit directly under, and hard against the floor, with the inlet in a place convenient to mount the toilet above it. The plumbing is simple once you have a suitable site. The toilet above dumps into the tank below, and the tank drains through the outlet valve down a waste pipe. No plumbing involved actually, other than getting water to the toilet above, which seems to be mostly by a foot operated pump from the caravan’s water tank.
If you import these parts yourself I reckon you’re in for around R15k to R20k, judging from the cost in the US and Oz.
Maybe the cassette type of toilet would be easier and cheaper to implement. Maybe that’s why they’re more popular as well.