Find a good paintless dent removal tech and have him assess the tank. Most car dealerships have a guy that comes by on a weekly basis to take out dents in new and used cars. If he can access the dent through the fill hole ,he should be able to take out the dent or atleast make it less noticeable without damaging the paint. As long as the dent doesn't have a sharp crease, the outcome is generally very good.