Problems with bladder?

Help! We bought our 04 Prius second hand about two months ago. Previous driver babied the car and had a very short commute to work. The car currently has 35K. This is the third time we have filled the tank and every time we only get about 6-7 gallons in it, although we are filling it from only one flasing pip remaining? This is the history of our fill ups in hopes someone can help me answer my problem. Tank 1 on 04/12/07, got 351.3 miles, cost $20. When I filled Tank 1 from empty, we only got 6 gallons. Tank 2 on 04/23/07, got 280 miles, cost $18.55. When filled from empty, only got 6 gallons in the tank before it was full. Tank 3 on 05/04/07, got 291.6 miles, cost $25. When filled from empty, only got 7.6 gallons. That means we're getting good MPG (47mpg on average), but how do I get more gas in there? How do I get it up to 400 miles per tank? We live in Northern California, mild to hot temperatures. Is there a problem with my bladder? With the computer reading the gallons in the tank? Don't want to take it in to Toyota to get the same run around we have had in the past. HELP US, PLEASE?

Thanks.