When we owned our Elddis I bought a Floe device to purge the water in the pipes. We have since bought a Swift and, of course, that has a different water inlet (a Truma vs. a Whale) and I didn't want to shell out for another Floe.
As luck would have it I bought a 3D printer last year...
As a result I created this using a tyre valve, two O-rings and some lovely blue filament. If you exclude the price of the printer *ahem* then it came to less than £5 including materials.
Should you have a suitable printer doing nothing then I've made the files available to download on Thingiverse
I used a cheap pair of digital calipers to take all the dimensions from the existing Truma inlet pistol and the car tyre valve that I'd ordered already.
I then created the object in Sketchup (which is free) and ran it through the slicing process to create the final file to print.
Unusually it all fitted first time.
The extra blue bit in the top picture is a clip to hold it in place just like the blue trigger on the Truma pistol, however the tight fit and the non-return valve in the water system means that there's not sufficient back-pressure to dislodge it.