1993 GMC Van EVAP problems 93 GMC G-2500 van

I have a 93 GMC G-2500 conversion van. I don't drive it everyday, as fuel is expensive, but I do use it for family trips, and for pulling heavy things like my 1993 Chrysler Town n Country to Pull-a-Part. (I did use a tow dolly). The problem I have had with it since I purchased it in 2005, is that there seems to be a whole lot of fuel vapor in the gas tank when I remove the gas cap. If the tank is fairly full, when I remove the gas cap a lot of fuel vapor escapes, but if I remove the cap when it is almost empty there is an enormous amount of vapor in the tank, so much so that the tank makes a popping sound when the vapor is rushing out, I gues the tank is changing back to it's regular size and shape. I thought perhaps an exhaust leak was warming the gas tank, and replaced all the exhaust from the cat back, but there is no change at all. The old exhaust was rusty and had holes, so it made sense, but it has not helped. I also have replaced the gas cap. So I really am looking for some direction as to why the fuel vapor is building up in the tank. If someone replies to my question, please have a pretty good understanding of GMC vans and the fuel vapor recovery system. Thanks.