Maybe it was just an oddness of his character, someone who really likes to rough it. Maybe it was a simple plot contrivance. If he'd been in bed and heard McKenzie, he could've leaped up and caught her. But instead he struggles to get out of his sleeping bag, and he stumbles as she's racing out the door.
I can think of plenty of plausible reasons why he is on the floor.
1) As mentioned, maybe he is getting used to roughing it.
2) Maybe being in a bed reminds him of his wife--maybe he'd rather sleep on the floor than sleep in a bed alone.
3) Maybe when he wants to pray or think he prefers to be on the ground--I don't usually lie down on the ground, but I will sometimes sit on the floor to think and just feel like I have more space above/around me.
But ultimately, yes, it is just a plot device for him to see her under the bed.
I kind of like the idea that he's down there because being in the bed alone makes him sad. Because him being down on the ground (sad because he's missing his companion) is how he ends up finding a new companion.
I have a friend with a bad back that always sleeps on the floor (yes especially hotel rooms) unless he can sleep on his hard as a board bed at his home. He will get terrible back pain if he sleeps on the pillow soft beds in most hotels.
I had not realized he was on the floor until I read this post, I had to watch that scene again. I thought he was in the adjoining bed and noticed her under the other bed. That's odd....