Well, I might as well get in here and throw a grenade.

I buy 5 @ 12V Wall Mart batteries that sell for $40 each and have an 8
year warranty. It is full replacement for 3 years and prorated after
that. They weigh about 40lbs each and are easy to move around.

Now, what if the one on the engine fails? Well, I have four more on
the house bank and any one of those can replace the engine battery
with a flip of a switch. Suppose I get 5 years before a failure
(typically thats what I get) and the proration has clicked in. Wall
mart is a short walk from almost any pier or at the least a short cab
ride if you cant bum a car from another boater. The price of the
replacement after proration is about $20 and you have a 40 pound
battery that you can easily replace yourself...I am 70 years old so I
know the young whipper snappers can.

Now, I can replace all 5 of them every 5 years and I will have only
invested $300 over a ten year time span. I will have invested only
twice as much time in replacement labor than the one time replacement
labor of the Rolls. Thats about 3 hours time. Thus I was paid
roughly $1200 for three hours labor (assuming that you need TWO Rolls
bats, one for engine and one for house). I would not run on one
battery no mater HOW reliable it is.