I have used the same batteries since I bought my boat 3 years ago. I have one cranking battery and one deep cycle. Because it was easier to connect my depthfinder to my cranking battery, that's what I did knowing I have shortened the life span of the battery.

So now it's time for a new cranking battery because it won't hold a charge for long. I want to keep my wiring the same and wondering if anyone has had luck with dual purpose batteries in terms of longevity.

What size is the battery you have now?

I have a 12V  800amp Interstate cranking battery.

A group 27, 30 or 31 dual purpose should work just fine for you.  I've been running the 27 DP on my Evinrude 225 DFI motor for seven years and it's never missed a beat.  Note: Not the same one, just the same size, I always replace the cranking batteries every three years in every boat and vehicle I own. 

Batteries are too cheap to put up the the problems one creates if it desides to die an early death.