??? about keeping refrigerators cold

Hi,

I have a large refrigerator on a houseboat, and don't want to
replace it with a less convenient one since the vast majority
of the time it's connected to AC shore power at the dock. I
like to go out for a couple days a week, and could *greatly*
reduce the amount of time the generator runs if I can find a
good way of getting the refrigerator to stay cold longer. Are
there any good cold packs for keeping refrigerators cold,
that can be re-frozen over and over again?

Thanks for any help!
David