Thetford recommends Aqua Kem (green or blue) for their cassette and portable toilets. This has a different function than the assortment of chemicals sold to go into waste storage tanks. Many of those waste tank products are not suitable for filling a bowl you will be sitting over and splashing into.
Work with the model number for the toilet to find an instruction manual. From your description of the other stuff in that van, it may have been equipped with some "global" fixtures. Some of Thetford's cassette toilets, particularly those used in Europe, might be recirculating, or have their own supply tank, rather than drawing from the fresh water supply. These have different chemical needs than one that just uses fresh water.