Sub for sure. Any trailer is going to increase fuel consumption that much more.
If your BOL is more than a tank away, you should find a new one.
This is what I was thinking as soon as I read the thread.
If you are going to bug out to another state via highway, think of every single zombie movie you've seen. Highways are clogged, you are **** out of luck. Even if the highways are free, you will be in traffic and have reduced gas mileage.
Your BOL should probably be within a comfortable 50-100 miles, any more than that and you are asking for trouble.
If you have to stop to refuel when SHTF, I hope you have tons of cash on hand, because I can see prices at $20 / gallon