Hi and welcome. What model year Horizon? Winnebago dropped the Horizon/Vectra line about 2007 and revived it about a year or so ago. There's several variables involved but a broad brush answer for a 40' diesel pusher towing a vehicle, the MPG would be in the low 7s to possibly low 8s. Our Gross Combination Weight (coach, trailer, Jeep on trailer) is about 40,000 pounds - we average about 7.2 - 7.3 MPG.
It's not cheap moving the coach from point A to point B - I figure about $0.50 per mile, this is just the cost of fuel.