Bing -- Over the years I've helped a number of friends do what you want to do, but mostly in campers and motorhomes. What you want to do is very common. If you go to an RV/motorhome center, like Camping World, they should have complete cable kits for connecting the batteries in parallel. I'm guessing they have similar kits, for boats, at the big box sporting goods stores, like Cabellas, BassPro, and Gander Mountain. I just couldn't find any by searching web sites.
Regards,
Ken
