I did read it somewhere, that water is allowed to put in gas at the gas station to a certain amount if there is alcohol in the gas they delivered.  Now, it is possible, that too many cooks spoil the broth and the last guy does not know if someone already put water and how much, thus more water is in than permissible.  Sometimes they just put more out of greed.  Usually this happens at gas stations by the freeway.  You won't come back.  Many times happened with me that a tank of gas started to do hick ups and jerkings with the car and the overall performance was bad, but the next fill up was fine.  

Now, if you stay in salted roads area, you can have the rusting out of the top somewhere and the splashed up snow and rain can seep in there and when filling up, leaks will happen there too.  Those tanks can be fixed with something I'm not sure, maybe JB Weld or change it to a new one or a better one.  

The leak by the grommets or rather the hoses and the fittings, are the most common.  Age related, must be changed.  
Takes all day for you two hours for me or less.

Take out the fittings and see which one is broken.  Get it from the dealer or one of our supplier, together with some hoses.  Now, these hoses don't need to be high pressure hoses at all.  There is no pressure of any kind there really, but they must be ones that are used for gas.  You can put them on with those plastic ties, preferably very small ones.  
I'm not sure, if anyone has the correct write up about this procedure but you can think it over and make a good plan, how to go about it.  It's a little bit of a puzzle, but not too hard to figure out which to do first and what to do to make it easy.

Enjoy playing with your ride.
Zoltan