It certainly can be frustrating trying to fill the water tank in a Vanagon
Westfalia simply by sticking the end of a garden hose into the inlet hole.
It seems there is a plastic partition with three of four holes in it backed
up by a rubber flap that covers the entire partition and all the holes.
When a water hose is inserted up to the plastic partition and turned on it
will only allow a small rate of water (maybe 1/2 to 1 gallons a minute,
thus taking about 15 minutes to fill the water tank which is more like
10gallons than the 13 gallons specified in the owner's manual) to enter the
tank fill tube. The problem is that the air outlet runs from the tank and
connects to the fill box on the tank side of the partition and rubber flap.
The water goes glug glug glug as the flap opens and closes to allow water
and air to pass each other. The result is that the air return is almost
useless because the partition and rubber flap are in the way, forcing the
air and the water to fight each other for access to the partition holes,
not to mention that any air pressure from inside the tank forces the rubber
flap to close against the partition!
My solution: Go to the hardware store and buy the proper fittings (I used
brass) to step down from 3/4 inch hose threads to male barbed tubing
fitting. Pick the size tubing such that the outside diameter just fits
through one of the holes in the plastic partition (I used about 6 inches of
clear water potable tubing and beveled the end to make it easier to
insert). Slip the tubing on the end of the male barbed fitting and connect
the 3/4 inch hose fitting to the hose. Insert the tubing end into one of
the holes in the partition, thus pushing the rubber flap back. Insert it
far enough to get past the air outlet (about two or three inches). Now you
can fill your tank in about 2 or 3 minutes with no spillage of water. I
also bought a cheap  on/off valve to fit into this setup just after the
garden hose so I can quickly turn the water on and off without having to
run back to the faucet. This set up has really worked well for me and saved
a lot of time and frustration and guess work as to weather my water tank is
full or not!!
Good luck... Bill Davidson... Westy Syncro '90