To expand on what "The Plotter" has written; a car battery is built differently - internally it comprises many thin lead plates designed to release hundreds of amps for a few seconds to turn the starter motor. Whereas a lesiure battery has only a few thick lead plates and is designed to supply five or ten amps for many hours.
Leisure batteries are also designed to "deep discharge" in that they will survive repeated cycles from fully charged to almost empty (never a good idea to fully discharge a lead acid battery), whereas a car battery won't.
Leisure batteries are (or should be) designed to recharge without "gassing" hydrogen and possibly acid vapours into the van - car batteries are not. However a special battery box with an external vent pipe can fix this issue.
If you have a spare car battery in good condition, your caravan has a battery box already, and you don't let it discharge below 50%, then give it a go. If you are thinking of buying new, then only fit a Leisure battery
Robert