Here is a link to a Xantrex XBM
Battery Monitor in use.
http://www.flickr.com/photos/76boler/2664448549/
This particular model has been replaced with a newer one. You might be able to find this one on sale on the internet or purchase the newer model.
It is nice to know the exact state of charge in the batteries taking into account variations in temperature and loads.
I purchased my monitor at JackRabbit Marine.
http://www.jackrabbitmarine.com/