Tank Filler Pipe And Cap problems of the 2008 Toyota Prius

I normally get about 450 miles out of one 11. 9 gallon tank. Recently, I discovered my fuel gage light going off at 300 miles. When I filled up my tank it only took 7 gallons, leaving 5 gallons that should have been in the tank. After filling it up, fuel gage light went off just below 300 miles. When I took my vehicle in for an oil change I asked about it thinking there was something wrong with my fuel sensor. I was advised by the clerk that it was the bladder, and to just keep it full-that¿s all. He gave other advise like make sure the tire pressure on my tires were good (they were rotated and checked a week prior to this), and to check my air filter. Now I am confused, and do not know how much fuel is actually in my tank. I have had my vehicle for 3 1/2 years and never had a problem till now.