I picked up an "E-Z Water Fill", made by Valterra, from my
local RV place.  It's similar to the water fill a few others have
described - attaches to a hose, and has a valve and a clear
plastic fill tube with an angled tip.  (Camco makes one of those
that's widely available; I used to have one until the valve broke.)
What's different about the Valterra is that the tube is narrower -
it's narrow enough to slip easily between the struts and through
the rubber one-way valve on the Westfalia water tank filler (on
my '88, anyway).  With the fill tube pushed past the valve, the
tank fills quickly, with no spillage (except that I usually wait for
the water to gush out before I shut off the valve).

I also use the fill tube as a funnel for putting in diluted bleach (for
cleaning) or Micropur (to keep the water fresh, from CampingWorld,
www.campingworld.com) .

My original Camco did OK too, but it was a bit more awkward to
shove past the valve.  Camco has a web site at www.camco.net,
with a picture of their filler.

Valterra has a web page at www.valterra.com, but there's
not a lot to see there.  They do say they're Y2K compliant, though,
so I guess my filler won't suddenly start spewing 100-year-old
water after December.

                                       -Steven Sittser