I have a ProMariner mounted behind the driver's seat.  It charges both
batteries whenever shore power is connected.

http://www.amazon.com/exec/obidos/ASIN/B001BSUR2Q/sbw/

It's not a particularly powerful charger at 8 amps, automatically
applied to whichever battery needs it most at the time, but it's
powerful enough to automatically go through the conditioning cycle.  It
has been working well for about nine years.

Here are the details of my aux battery and volt/ammeter installation:

https://sbw.org/westybattery/

At the bottom is a note from Karl Mullendore recommending a more
powerful Iota charger, which I might install at some point.