Since the thread's revived anyway...
Just curious here fellas, but why not keep the deep cycles split into two banks for redundancy and use a separate start battery without wiring it to the onboard charger at all ?
Either a two- or a three-channel isolator in the alternator's output will take care of separating the start and house batteries, and I would expect that motoring out of the marina will be more than enough to replenish the start battery. For the start battery to self-discharge would take quite a while.
It would be easy enough to wire a parallel relay between the house banks' battery switch and the starter just in case the start battery is flat (did I mention I like redundancy? ;^)
"Verbosity leads to unclear inarticulate things"