I would bet that you need the correct petcock. At those speeds you mentioned, you're sucking a lot of fuel. I remember my first trip on my 1300. Around here I routinely got around 45mpg. After my first fuel stop on the super-slab, I was only getting 33mpg. Doing 70-75 mph which is loafering today.
Take a peek at Partzilla.com and look at the parts fiche. Compare your tank with a newer model (05, 06). I know the petcock assy is costly, but you might find one on Evil-bay -- just to try.
I messed up my original tank and got a second hand one. My friend and I took a sawzall to the old tank, cut it in half. I wanted to see what was inside
That vent line was really hard to understand. It was routed from the rear of the tank in an "S" pattern along the top of the tank to a small rectangular metal box at the front of tank in front of the filler neck. Looked like a solid box to me......strange vent system 
Long winded post here, but wanted to share.
tom
Take a peek at Partzilla.com and look at the parts fiche. Compare your tank with a newer model (05, 06). I know the petcock assy is costly, but you might find one on Evil-bay -- just to try.
I messed up my original tank and got a second hand one. My friend and I took a sawzall to the old tank, cut it in half. I wanted to see what was inside
Long winded post here, but wanted to share.
tom