When you want to start an engine, espically on a cold winter in the north morning, then COLD CRANKING AMPS are good, you need lots of them
Thus starting batteries are long on cranking (peak) amps, even when cold, but short on Amp Hours
When you want to listen to the radio, watch TV, read a book or just pump heat or water, AMP HOURS are good, you want lots of them, Thus Deep Cycle batteries are long on AMP HOURS, but short on cranking amps.
MARINE batteries are half breeds. They are long on nothing and short on everythign (Medium actually on everything)
Example. One popular size 12 volt battery is 68 AH starting, 83 Marine and 100 DEEP CYCLE
Marine are often called "Marine/Deep Cycle" but any battery that ilsts "CRANKING AMPS (CCA or MCA) on it's lable is not a True Deep Cycle
Six Volt V/S 12 volt:
Many folks say six volt is better, NOT TRUE, first there are no six volt batteries in RV's, Just 12 volt batteries that come in two pieces (Think of each pair of six volt batteries as one 12 volt and many questions self answer, Example "Do they both have to be the same?" A: Would you buy a 12 volt battery that was half interstate and half excide?)
There are a lot of golf carts out there, and thus a lot of companies make six volt golf cart batteries (Cause that is what Golf Carts use) and thus due to both the size of the production runs and the number of competitors prices for Golf Cart batteries are low in compairson to other types of Deep Cycle batteries. At one time in fact about the only reasonable size TRUE DEEP CYCLE battery you could get was a golf cart battery (Other Deep Cycles were made for things like Fork Lifts and such and.. Well, you need a fork lift to lift a fork lift battery, Trust me on that, I've use one)
So RV makers used golf cart batteries
Today due in no small part to the size of the RV industry, more and more we are finding 12 volt DEEP CYCLE batteries in sizes you can pick up and carry (24,27,31,and the like) up to about 8D. (much bigger and you are gonna get a herina, Been there and done that too)
And the prices are dropping... But I think Golf Cart, at around 50 cents per Amp Hour at Sams or Costco, are still your best bargain, 12 volts are around a buck an amp hour just now.