Motorhome RV's usually need to have two batteries, while RV trailers only need to have one. A motorized RV will have a separate starting battery for the chassis, the same type that your car or truck has in it...and there will be at least one more battery in order to supply 12 volt power to the "house" portion of the RV.
The battery that runs the "house" portion of the RV and comes with the RV when new is usually a marine starting battery. These batteries are not a true deep cycle battery, but are good enough for the person who only needs to worry about getting plugged in at the next RV park they get to.
These RV marine batteries are only designed to be drawn down to 50% of their capacity. A fully charged battery will read about 12.7 volts. At 50% state of charge, that same battery will read about 12.1 volts. The more times that you draw one of these batteries below 12.1 "resting" volts, the quicker that battery will die.
These 12 volt marine batteries are made up of six 2 volt cells. Because there are six cells in the battery case, the lead plates have to be fairly narrow.
Most true deep cycle batteries are 6 volt batteries. These batteries are made up of three 2 volt cells, but the battery itself is approximately the same size as the 12 volt ones. But because there are only three 2 volt cells in the one battery, the lead plates are made much thicker, and can withstand a deeper drawdown (up to 80%) and many more cycles of charge and discharge.
