I run my bowmount motor off 2 deep cycles at the front of the boat, and everything else, including 2 depthfinders, a radio, bilge pump, all the lights and all the guages off the cranking battery.
I have an onboard charger for the 2 deep cycles, and the cranking battery is charged by the engine alternator.
All 3 batteries are 7 years old, get used ALOT, and are still as strong as the 1st day.
I think the most important thing is to be sure to always keep them fully charged. If I use my bowmount more than 30 minutes a day, I plug it in when I get back to the dock.
And BTW, I've NEVER used distilled water in my batteries.......just good ole tap water and have never had a problem......touch wood