6 volt deep cycle batteries, often called golf cart batteries, are designed for many low amperage longer term discharges. 12 volt batteries are generally designed for high amperage short term discharges. It has to do with the design of the plates inside the battery.
Your car uses a short term high amperage draw to crank the motor then immediately recharges via the alternator. For this use a "starting" battery is appropriate. Your RV has some lights, a fridge, maybe a tv, etc. These items use relatively small amounts of amperage over a long term so a deep cycle battery is appropriate.
There is a compromise battery, a 12 volt "deep cycle" or more often "marine" labelled battery. These are a compromise between a true deep cycle and a cranking amp battery.
Most RV'rs use the 6 volt true deep cycle battery for their "house" bank. And will have a "chassis" battery to start the engine and run the chassis items. For trailers/5rs/etc with no engine of their own, a house setup, deep cycle, is appropriate. Depending on the size of the RV and its specific use, there can be multiples of each battery type. On my MH for example, I have two 12 volt starting batteries (wired in parallel) for the chassis side. On the house side I have four 6 volt batteries (wired in series/parallel) for the house side. Other rigs may have 8 or maybe even more batteries on the house side.
Since we don't know anything about your RV or proposed RV, a general explanation is all we can provide.
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