Hello Raymond.
I would be very surprised if any caravan shower could deliver 7.7l/m. That is 1.7 Gallons a minute, and that would empty a water heater in about 1m10s. of undiluted heated water.
From work i did over ten years ago, I recall the flow rate at caravan shower heads was at best about 3l/m and more typically 1.8 to 2.2 l/m.
Anything you put in line with the pipe work will add some resistance to flow, and reduce the flow rate further.
as Jasper writes, its free, but frankly I cant see any point in trying it as it is likely to reduce the already meagre flows caravanners already get.