2003 Toyota Rav4 4 cyl Four Wheel Drive Automatic

Hi. I have a 2003 Rav 4. First, I don't get very good gas milage on it for some reason. I can barely get 300 km on a full tank of gas. I used to have a 1998 Honda Civic SIG and I could go over 450 km on a full tank!! Anyway, I changed the air filter, sparkplugs...no difference. So, the other day, i parked the Rav in the garage and I'm hearing some funny noises. Like a low burping sound. I walked around the Rav and found out that the noise was coming from the gas tank. I unscrewed the gas cap and heard alot of air coming out. Is this normal?? The burping sound and the air in the tank?? Could this be why I am getting such poor gas milage??
Please Help!
Thank you!
Gail