I think the dog might be part cat! He slept all day in the car on Saturday, rousing himself occassionally for walks around rest stops.
Then it was back to sleep.
We got to our hotel and after a walk and dinner, he was back to sleep again and stayed that way all night.
Sunday he repeated this. He has slept about 20 hours a day for the past two days and seems none the worst for it.
Just like a cat. I should have named him Kitty.