The thickness of the tube plays a huge role in how quickly air leaks through it. Most tubes you find in a shop are the thin racing types. On request, you can pick up a much thicker touring-type thorn-resistant tube. The thickness of the tube on the outside tread-side can be 3-4x thicker than a racing tube. The other rim-side will still be 2x thicker. This will keep the tyre up to pressure for much longer, easily less than 5psi loss per week. Heck, on my ride across the U.S. in 1995, if it wasn't for the 2 flats I got, I may have topped off my tyres at most 4 times during those 6-weeks.