This is a fairly common occurrence with the old Beetles. Some good places to start looking are the filler lines from the point where you put the gas in down to the tank itself. The couplings being old tend to seep especially with a full tank. Up under the cowl you may find a long plastic gadget with lines going down to the tank. This was originally a device to deal with polluting gas vapours. As they get old and brittle they become a primary source for gas fumes. Do yourself a favour. If you do have one there, yoink the damned thing off and plug all of the lines with short pieces of new line plugged with suitably sized bolts dipped in Neverleak. This stuff comes in a red and yellow tube. It gels in the presence of gas and is really great stuff even for actively leaking tanks. Give this a try and see if it helps.
I'd rather have a partial bottle in front of me than a partial frontal lobotomy.