If you have lots of money I know the perfect solution for you, search on
google for maxpower or fuel cell marine and you'll find a small automatic
totally quiet fuel cell that will deliver 400 Watts for 24 hours a day. I
saw one at a boat show in Florida a few months ago and it does just what
you are looking for. I can't recall the fuel that it used right now but
running at full load it could run continuously for about 3 days. The down
side is the cost, it was about 5000 US so you might want to wait a while.