Other Fuel System Hoses Lines/piping, And Fittings problems of the 2008 Hyundai Santa Fe

For past 2 years my fuel tank will sender it's full when it's been low and only too 5 gals in a 19 gal. Tank. I have read over many times complaints that the gas gauge doesn't work?? I've been quoted $425 to repair it? it's a 2008 Hyundai sante fe limited. The chk engine light was on my mechanic used the car detector system. It states fuel cap issues. I replaced that $70 later it not working. I've dealt with this for too many years.