I thought that Formaldehyde added the breakdown of solids?
Not an expert, but I do know that formaldehyde is used in hospital pathology departments to preserve samples as they are at the point they're fixed, so in that context at least they're preventing decomposition.
That said, I've never known a caravan toilet chemical, of whatever colour, that significantly breaks down...umm..solid matter...in the short timespan between the matter entering the tank, and the tank being emptied. Waste stays in the site sewage pit for much longer, obviously, and that's got to be where the break down happens.