Cheap bio liquid. Presently Sainsbury's basic but Tesco's equivalent is equally good. I find both better than Aldi or Lidl so use either whilst in the UK.
In winter time or when the temperature is low I sometimes don't bother with additives, especially when there is no or little solid waste.
We use the same green sachets now.It was always the thetford blue stuff until we stopped on a club site which only allowed the the green stuff to be emptied into the septic tank.I bought some of the sachets from reception and we have used them ever since,preferring them to the blue.Another bonus is that they are in sachet form so no measuring of fluid required.
Just use the blue stuff. About ten quid for four litres which lasts for about five months continuous usage. I tried the various tablets and stuff but nothing was as good. Maybe my poo is nuclear or summut .I dunno.
I did read somewhere that when changing to using bio it takes a few uses before the blue is eradicated as any trace of it kills the bacteria that do the work. It's also good not to rinse too thoroughly in that the chlorine in the water can also kill the bacteria.