2001 Sportage Convertible- Multiple Problems

Hi. My sister has a 2001 Kia Sportage convertible. When she got the car we knew that it got horrible gas mileage. The car was maintaned because we have all of the records so we don't know why the mileage would suck. It only got about 200 miles on a full tank. The belts always sound loose and are annoying while driving but the Kia dealer says that this is normal for it to sound this way. Recently, the car started to idle kinda hard and my sister said it shakes more than it normally should. I checked the oil and it was really black and looks like it needs to be changed so we are going to get that done this weekend. So, a new problem this week also arose...the gas mileage has gotten worse and the gas light came on at only 150 miles after filling up to full. This sounds absurd since my 10 year old 1995 Geo Prizm goes 300 or more miles on a tank of gas and I sit in way more traffic than she does. What do you think this could be and has anyone else experienced these problems? Also, the rag top on the convertible makes a lot of sound while driving, almost like its raining. This car seems like a victim of very poor engineering. I knew we shouldn't have gotten her a Kia but I hoped I would be proven wrong. Too bad.