I had the EXACT same problem as described in the original post, only at about 60,000 mi. I never heard about a gas additive causing problems, but I suppose that could make sense. I was not able to repair the broken sending unit, as the "fingers" on the level float were all broken off.
Took about 45 mins to remove gas tank. From there the sending unit is a screwed on cap on the top of the gas tank. Removed and replaced sending unit (new sender about $75 from dealer; BONUS you get 2 in the package, but with different connectors. You could remove the connectors that DON'T match yours and alter that sending unit to work with your car for next time).
Total repair time; about 2 hours.