Gas Smell in E-350

I have this recurring smell of gas after I shut the vehicle off. It is worse on short trips (5-10 min).

I have had the van in the shop 3 times and after paying for a carb rebuild it is still present though not as bad. THe mechnic said that it could be a sarurated charcoal bed in the recapture system. I do not know where or what the Charcoal bed is and I do not want to spend more money at this shop so trying to research this my self. Any ideas??

Thanks, Mel