Re: 5 miles to a gallon
Have you observed the bike while parked with the petcock OPEN? I had a bad float in the carb on one of mine that caused the gas to flow out of the carb and onto the ground. If you are very judicous about always turning the petcock off when not in motion you may not notice the loss of fuel this way as it's spread out in little drops along your path like bread crumbs.
In my case either I or one of my son's (don't remember who rode it last) left the petcock open and I discovered a nice puddle under the bike.
The fact that you have NO leak when the valve is closed points to a leak further down the system.
Like another poster I also had a tank that leaked where the tank bolts attach to the tank. JB Weld solved that one.