Where is it? Does it even exist?

You could of course fill a small spray bottle with a dilute solution of pink and spray the toilet after use and before you flush. There are those on CT who use this method in place of a flush at all.

Just regard it as a potential saving. I found the pink stuff gave me black stuff in the toilet so never use it.

We use pink all the time and have had no black stuff, perhaps because we're out in the van for more than 250 nights each year, so the flush tank is regularly in use, and the black stuff doesn't have chance to build up.

.... I now want to add rinse to my system

You can't 

Our previous van had no separate flush tank, we had no problems with that.

Now we do have  a flush tank, even after 8 years we are still surprised when it runs empty!

We hardly ever use anything other than plain water in the flush.

We did buy some Elsan pink about 4 years back, but keep forgetting to use it, once it is finished we will not be buying more.

(We are away about 4 months per year)

You could put it in your aquaroll, it will reach the toilet eventually 

Thetford are a very wise company and in manufacturing a system that prevents me from buying their toilet rinse ( which I wouldnt anyway ) and adding it to my sytem seems to make poor business sense.

Thetford have thought of that, they sell a spray to deodorise the bowl.

Separate flush tanks in motorhomes seem to have disappeared about the same time contnental motorhomes started appearing in numbers on our shores. Last time we had a separate flush tank was on our Swift in the 90s. We mix our own pink rinse in a spray which we use occasionally as a freshener after a normal flush. The Thetford spray is very expensive for what it is.

We used the pink stuff, and found the bowl was slowly becoming stainerd yellow.  So, after reading on here that someone recommended a bottle of 'Comfort' from Aldi's, we use that in the flush to keep it fresh and I made a diluted citrus liquid in a spray bottle and use after each flush on the bowl to clear the yellow stain (which I'm assuming was limescale build up but could be wrong.)  Seems to work

Just a thought! Would you put "pink stuff" into your toilet cistern at home?