What makes you think you get better mileage if you let the tank go to empty? How much better is it?
While I'm glad that you're pleased with the mileage, getting 300 miles in a tank isn't much of which to be proud. Jeeps just aren't known for the good mileage, and even with a 20 gallon tank, it still doesn't get many people to their destinations in one tank. In my old Ranger, I could go from Los Angeles to Tahoe and just barely make it with a 20 gallon tank of gas, and that's nearly 500 miles.
As you know, stopping for gas is a hassle. I really don't mind the mileage, but a bigger tank would be nice, even if it does cost a fortune to fill.
Jeep: 2006 Midnight Blue Grand Cherokee Limited 4.7L Bells Whistles
Mods: Tow Hooks Mopar & 4xGuard Skid Plates & Rock Rails Flowmaster 70 255/65R17 REVO's