My HP16 has a forward and aft tank in each wing for a total of 4 tanks.
on the Root Rib I have a Outlet fitting and a vent fitting for each
tank. The left wing has an outboard fill port on each tank. I have a
single Dump valve in the bottom of the fuselage that was homemade and
has two 1" vinyle hoses, connected to Each Hose is a Tee with hoses
going to the forward and aft tanks. BTW the Tanks are made 4" PVC
Tubing. I think one tank is slightly smaller maybe 3" tubing? I can
carry a total of 21 gallons.

The vent tubes are all Tee'ed together with 3/8" vinyl Tubing in an H
type fashion, Then the center of the H also has T with the 3/8 vinyl
Tubing that is looped up through a ring in the top of my Turtle deck
and then back to a 3/8 aluminum tube that is mounted in my gear well
to drain any overflow. I drill an 1/8 vent hole in the vinyl tube at
the high point of the vent system where it passes through the Ring.
This prevents water from siphoning out the vent tube.

to fill this system put the right wing down, open the fill ports on the
left wing and begin filling it does tend to burp quite a bit for the
last 5 gallons or so, so the fill rate has to be fairly slow. I usually
plug the Vent line while filling. It takes me about 20 minutes to fill
that tanks. It takes about 10 minutes to dump all the water.

The disadvantates of my system a

1. about 8 hoses to hook up after installing the wings. Much of the
time I fly dry, and just leave them disconnected.
2. The outboard end of the tank is just about he same hieght of the
top of my turtle deck when one wing is on the ground, so I have the
keep the wings somewhat raised to keep water from draining out the vent
tube.
3. if the tubing comes loose you can end up with a cockpit full of
water. It has only happened once but flying in the bathtub is a bit
annoying.

If I were designing a new system I think I would try to keep the tanks
and vents all separate or at least contained in each wing so that there
were a few connections to make as possible, after assembling the
aircraft.

The Vent From the tip back to root seems to be best solution I have
seen so far for venting, with least # of disadvantages..

Brian