Safe way to transport dogs in Motorhome

We will be taking Brother and Sister in law and their dog touring Yorkshire  for ten days in the near future. What is the best and safest way to transport the dog in our M'home?(M'home on avatar) I thought maybe a stairgate to section off the cab area, but should the dog be allowed to stay free in the back, with its owners? The dog is well behaved and average size (lab cross) All advice welcomed as we have never taken  an animal in our  van. Thank you all in advance...