Wagon for road trips (high mpg, sleeping room)

I live in the US and due to my work schedule often travel long distances by highway at night, sleeping in my car at rest stops during the day. I used to drive a 2000 Chevy Blazer ZR2 but I'm a big guy and don't get a good day's sleep if I can't stretch out. My current truck - a 2007 Chevy Tahoe - is the perfect size but is getting less than 15mpg highway, even with cruise control at 65mph for 4+ hour stretches. I didn't buy either of those cars with the intent to use them like this, they're just what I had at the time. I'm looking for something that is going to have the length to sleep in (75"+ flat, not diagonal), gets better than 30mpg highway, and is simple/reliable. I have no current budget numbers in mind but I am definitely in the used market. I actually prefer older cars as in my experience issues tend to be less costly to fix, especially when on the road very far from my usual mechanic. The only car that has so far checked my boxes on paper is a 2007 Ford Focus wagon, though I don't have any experience with them. My old 08 Focus sedan was very reliable and fuel efficient but I don't know how roomy the wagon would be.