Not sure where your leak might be coming from. Could it be seeping from the cap seal and getting inside the wing from there?
I have never folded the wings with more than a few gallons per side. Too much weight on the tail and fuel will drain out of the vented caps even if close to being full.
Some have made a tailwheel jack that levels the plane and still allows it to be moved around while fueled.
Hope you find it! Here's some links to help.
https://teamkitfox.com/Forums/thread...lding-question
https://teamkitfox.com/Forums/thread...-Folding-Wings
https://teamkitfox.com/Forums/thread...l-in-the-tanks