Electric motors (possibly with solar panels) might do the trick.
The problem when using those on a plane is that the battery does not get any lighter as it drains, while fuel tanks do. This means that as the plane goes farther it also gets ligther and gets a (slightly) better mileage.
But since you don't need to use the power for lift in a zep/blim/balloon this drawback isn't so bad. In fact, I would see that zeppelins(and things like them) will become more and more affordable in the future.