The oversimplification- the deep cycle battery is designed to be charged slowly, and then discharged slowly. As opposed to your car battery which is designed to deliver a whack of power at once to start the engine. The car battery can also take the whack of power back.
My "Plan D" for dry camping does involve charging my RV battery using jumper cables from my running vehicle. However as mentioned, that is a last resort as doing that frequently is not good for the lifespan of the battery.
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