Should I Add Leaf Spring or Air Bag to Suspension

I have a 27' 2005 E450 Class C chassis. Sometimes going over a steep driveway or dip, the rear end gets so low that it scrapes the steel drag guards. While it doesn't happen often, it is irritating and sometimes causes a jolt. I think raising the chassis about 2" will eliminate 90% of the scraping. After reading RVnet comments, I thought adding another leaf spring might do the job but when I took it into a shop, they suggested adding an air bag instead. I don't know if they made the suggestion because adding a spring would be too difficult or too expensive or that an air bag really is more effective for situations such as this. Any comments would be appreciated. Would especially like to know if anyone has done this, does it work as planned and what cost was. Thanks.