Marine deep cycle batteries are not deep cycle batteries. They are merely deep
-er cycle than are cranking batteries. They are a hybrid battery, a hybrid between cranking and a true deep cycle battery. The marine deep cycle batteries of the type found at Walmart (a.k.a. cheap) are designed for the deep cycle drone-draw of a trolling motor, generally speaking up to about a 5 amp draw for various time periods. True deep cycle batteries are designed for higher amp draws for extended periods.
Cranking batteries and hybrids have "sponge" lead plates, where the plates are full of holes to get more surface area for the lead acid paste. True deep cycle batteries are insanely heavy and have solid lead plates.
There are cheap-ish deep cycle batteries that are labeled "AGM Deep Cycle" that are AGM batteries with sponge plates. Even though these are hybrids insofar as the lead plates, they are light years ahead of the standard "marine" deep cycle hybrid found at places like Walmart, Autozone, etc.
True deep cycle batteries which are AGM or gel aren't cheap by any means, but if you take proper care of them (correct charging regimens, proper monitoring, etc.) they will last 7-10 years or more. True AGM deep cycles include Concord Lifeline, Discover Energy, Odyssey, Deka and Trojan now has 'em, too.
If you want a cheap marine hybrid, the same kind that have the same charging requirements as your cranking batteries, then the ones Walmart, Autozone, and the like are what you want. Better AGM hybrids that need a little more particular charging regimens can be found at Sears, Cabela's and a few other places. High end (and high dollar) deep cycles that must be charged under far more stringent parameters can be found at
THE SOLAR BiZ, Espar of Michigan, Batteries R Us, lots of places.