It requires a battery that is capable of providing 12 volts, most put the largest battery that can fit in the battery compartment.
Whether it is just a car battery or a battery more specific for RVing is dependent on the type of camping they are planning to do.
A regular 12 volt car battery will work fine if they just plan to use their RV at RV parks where power is supplied.
If they plan to do lots of camping where power hookups are not available a deep cell golf cart type of battery lends to better results.
Unlike cars and trucks towable RVs are not designed to use a specific size battery, usually the dealer installs a basic 12 volt car battery.