First you need to take the cover off compleatly.4 spire clips on each pivot.
Place it somewhere that won't cause scratching, I do mine on the lawn.Turn it over carefully & tape over the pivot holes. Then fill the upturned window withhot soapy water & allow to soak a while. Then , using along bottle brush you can clean between the 2 layers. A hose around gets rid of most of the rubbish.
Carefully stand iton end to drain & refit.
You may need new clips that are10 mm & can be found on fleebay. I got mine from a guy in Germany,
P M if I can help further.
You and almost every other Motorhome owner must wonder, from time to time, who designed a window that becomes brittle and cannot be cleaned. Me thinks Fiamma have a nice little earner. Over the years I have replaced three.