Picture is worth a thousand words.

This is a 3-line tank.

Line 1 is the clunk--that is on the left side of the stopper in these pics.
Line 2 is the vent--thats the right side line
Line 3 is the filler--thats the one on the bottom of the stopper in these pics.

Take your vent line and run it up around the back of the tank, then run it down and out the bottom of the plane.

You can see in these pics that I have 2 tanks. 1 for gas and 1 for smoke oil. I ran the vent lines side-by-side and wrapped them back around the tanks and then made a little lite ply bracket and used some CA to attach it to the bottom of the plane. Put a couple brass tubes on the vent lines and epoxy them in. Makes a clean setup. Your tank is full when the vent line pukes out fuel.

The filler line just goes in the tank and makes a turn straight down to the bottom of the tank in the front. I used a clunk on them. The fuel line on this filler line is only about 1" long, and then it connects to the clunk. When I want to completely drain the tanks--tip the plane up on it's nose and pull the fluid out.

The reason why you do it like this is because the fuel won't leak out--no matter how theplane is oriented. You can have a full tank of fuel and tip the plane on it's nose without leaking fuel. The fuel would have to run uphill in the vent line and around the tank and then back down again to leak out. Highly unlikely.