My solution -

The housings in the type of filler pictured have an air vent hole at the top and so should allow filling at a fast rate.  Should, but rarely do due to poor fitting and routing of the hoses.  In my caravan 2 litres per minute is the fastest the system could handle!  With my home made click adaptor filling is now 10 times faster.

This adaptor consists of a fuel tank cap with a hose click fitting mounted to it.  The click fitting is the type designed to be retrofitted to older sprinklers and has a narrow 1/4" BSP thread that is small enough to fit inside the fuel cap threaded section.  A standard hose click fitting typically uses in water filters has to big of a thead to fit.

First step - get the parts:  1) Sprinkler Adaptor,  2) Nut to suit,  3) 44mm Fuel Tank Cap.

Fuel caps with 44mm screw thread are readily available and '6mm Sprinkler Adaptor with BSP thread' is easy to find at any hardware store for ~$4.  But 1/4" BSP nuts with 1.8mm per thread spacing are very difficult to source in metric Australia.  Luckily for me I had one in my jar of old nuts and bolts in my garage.

You'll save lots by getting a cheap fuel tank cap from ebay, like this one - "Angstrom Autohaus - Hard Plastic Oil Fuel Tank Cap Spare Part Assembly", $8 delivered.  (or pay $30+ locally).

You will also need a screw driver (flat blade), hammer and a nail, 1/2" drill (or smaller + file), pliers, sealant (optional) and some means to cut the soft plastic cap.

Step 2 - Disassemble the Fuel Cap

To separate the Black outer cap from the White inner just squeeze the sides along the line of the grip bar to distort the Black cap enough to release the clip ~75° around.  This will allow a flat bladed screw driver under the edge of the White part to lever it out.

Step 3 - Remove the Vent Assembly

Turn the White inner part threaded side down and use a nail to tap out the vent assembly.  Aim a little to one side, otherwise you will just put a hole in the rubber seal.

Step 4 - Making Holes

The plastic is very soft.  I drilled the hole in the White piece to 10mm and used a round hobby file to enlarge it for the 1/2" thread.

For the Black cap I just snapped away with the wire cutters to remove big chunks and again finished with a round file.  Have a hose fitting handy to check for an easy fit.

Step 5 - Assembly

Screw the click adaptor in place - I used some glue to seal and lock the thread.

Press the White screw plate into the Black cap.

Wait overnight for the glue to set.

All done.

Notes -

My nut was brass, if using steel nuts then some paint may be needed to prevent rusting.

This adaptor is a filler, not a permanent cover.  A blanked off click connector could be used to seal the tanks, but I prefer to use the lockable covers.

The water tank plumbing in my caravan must be truly awful, even with this adaptor I get water (not air) coming out of the vents from when the tanks are only 1/3 full!  And I still have to turn the tap down a bit to stop back pressure leakage around the cap seal.  But filling 2 x 90L tanks in less than 10 minutes is much, much better than the 90 minutes it used to take.

This works for me, ymmv

BSP is British Standard Pipe thread that is used internationally, except in America where (almost identical sized National Pipe Thread (NPT) is used.

BSP is the same as BSW - British Standard Whitworth thread, except that the sizes don't match up.  BSW 1/4" is 1/4" diameter but BSP 1/4" is actually 33/64" which is the outside diameter of a standard steel pipe that has an inside diameter of 1/4".

1/4" BSP is 12.7mm male thread at 1.34mm per thread.  Nuts are 18.6mm across the flats & 21mm across the points.  No metric nut is anywhere near to this specification.  American 1/4" NPT is a little bigger than BSP, but close enough to fit.