Hi Hell-Fire,
I had a similar sounding problem on my RFSV RHS fuel tank. It leaked when ever I filled up to full, or went over rough ground/terrain.
After taking the front seat off, and removing the top cover, I found that the sender unit was loose. Took it off, cleaned the fuel tank top (neck) and the rubber seal and the flanges, etc. and then fitted firmly back on, making sure the top fixing-flange was firmly screwed in over the rubber washer & sender unit.
It has been OK ever since.
Hope yours is as simple as this too
Good luck.
Muesli.