Here's a sit I stumbled across, thought I'd do a quick search before posting.
Deep Cycle Battery FAQ
Have a read and do even more research.
Not sure what your CCA requirement is, but if you plan to go from a starting battery to alternator delete and a deep cycle, plan to go at least 2, if not 3 times the weight to do it properly.
I would not advise going two 12V in parellel as when they, and they always do age and become unequal the can go into cyclic charge and discharge of each other till both batteries are flat, otherwise you will need to include high amp isolators so batteries are disconnected when engine or charging is off. A better system would be to use 2x 6V deep cycle batteries in series, still giving 12 V, but no need for isolators, if one fails you replace it, still ok to continue with two unequal batteries, just maximum current is governed by weakest battery, but doesn't affect efficiency of system.
You probably need to extend your research into the traction battery area/ stand alone power supplies.