I am on the road all the time. I have a concierge service that even though most will argue it is just an out call, I don't know many that will out call to somewhere 3 plus hours away with a five hour notice.
Anyway, I get around needing a travel fee by requiring a 2 hour minimum session for any session more than an hour away. The guys get that it has to be worth our time. If I am able to book extra sessions (I have a list of "If you're ever in my area" guys), then it is a bonus day. I don't offer any discounts when I am traveling at a single clients request, even if I book an extra session.
I have been doing this for over three months now and it is very successful for me. Of course 36 L boobs don't hurt in the equation either.