First, let's talk about their security in the vehicle. With dogs, depending on their size, it may be feasible to leave them unfettered, as long as they are inside the vehicle with their people. Some folks elect to put them in Kennel Carriers. That may be a very good idea, for the carrier may then be tied down with seat belts, keeping the pet relatively safe in the event of an accident. As far as cats are concerned, I would never let them remain loose in any vehicle.
Cats tend to be curious critters and often end up under the driver's feet.
Hit the brakes in a hurry and squish the cat. We always keep our cats in their individual carriers, strapped down with seat belts. Most RV parks don't make any such distinctions regarding cats, however. Well, for one thing, cats don't bark. Secondly, they don't act in an aggressive manner toward people who are walking about.