There are balls on the market which allow treats to fall out as the dog manipulates the ball and others into which a treat can be inserted encouraging the dog to lick or paw to get the treat out. These can keep a dog busy for quite some time. Just make sure both the toy and the treat are safe. Make sure your dog is familiar with your RV, especially its sights and smells. Leave some articles about with your scent on them.
Some pets suffering from separation anxiety are destructive.
Crating, however, does NOT take the place of training your dog. Don't leave pets unattended for unreasonable lengths of time. And, finally, we must realize that, like people, some pets just don't make good travelers. If you have honestly tried the methods above over a length of time to no avail, perhaps your pet would be happier at home. There are many ways to train a dog.