When we tried filling the tank from the outside, it seemed like most
of the water flowed down the side of the vehicle instead of into the
tank.  We constructed a filler gadget from a plastic hose connector
with a valve, a replacement plastic female hose connection, and a
7 inch piece of tygon tubing.

The tygon tubing fits over the nipple of the hose connection and is
secured the with a clamp.  It is attached to the male end of the
connector with the valve.  When it's time to fill the tank, the female
end of the connector is attached to the hose.  The tygon tubing fits
inside the intake to the tank, which keeps the water from pouring
down the side, and the valve allows for easy on/off.

We also replaced all the water tubing (some of which was really
quite black inside!) without pulling out the cabinets.  The only really
tricky part was the hose behind the cabinets.  We cut the old hose
to the sink on an angle that was about 2 inches long and did the
same for the replacement hose. We taped the two hoses together
along the cut with electrical tape. This allowed us to use the old
hose to pull the new hose through behind the sink/stove cabinet
(there are two round clips that the hose runs through behind the
cabinet).  We had the external tank inlet off at the time as we
cleaned the flexible hose going into the tank with a bottle brush.
Actually, getting that back together again was an interesting
exercise as well.