If there is no gas station, grab a cup of coffee at McDonalds. Or get a $5 souvenir at that hole-in-the-wall place. Or a candy bar at a 24 hour Walgreens. Or grab $20 from an ATM cuz they usually have really good receipts. There is probably something at a corner you can get.
I typically plot out my route on Google Maps. If I have to drag the route around, whatever point I dragged the route to is a corner that should get a DBR. If Google Maps automatically plots the route I want to take by just putting in my start and end points, I don't worry about corner receipts. There is really no corner to document. I also have Spotwalla tracking me while I ride so that helps too.
If I start in Houston, then go to Dallas, then over to Jackson, down to Baton Rouge, then back to Houston, I'd get one at each of those cities when I switch from a north/south interstate to an east/west one. (I don't know if that's a good route or not. I didn't look up the mileage, just assumed based on what I know of that area of the country. I think that makes a decent square shape.) For an out and back type ride, I try to stay on a single interstate and just make sure I get a good receipt at the turnaround point.
Edit* Oh, good luck on your ride.
2nd Edit* I was curious, so I mapped out that route. 15.5 hours and 1075 miles all on major interstates. It's actually a pretty good route for an SS1000 as long as you don't hit Houston around rush hour. I-45 northbound can be an absolute nightmare!