* Year of the van: Older models will generally get worse gas mileage than newer ones.
* Engine size and type: A larger, more powerful engine will use more fuel. Gas mileage will also differ between gas and diesel engines (diesel typically gets better mileage but is more expensive).
* Weight: The weight of the camper van itself, plus the weight of any cargo, significantly impacts fuel economy. Heavier loads mean lower gas mileage.
* Driving conditions: City driving, hilly terrain, and high speeds all reduce gas mileage compared to highway driving at a steady pace.
* Maintenance: Proper maintenance (tire inflation, tune-ups, etc.) improves fuel efficiency.
To find an estimate, you'd need to specify the year and engine of the particular van. Even then, it will only be an estimate. You're more likely to find real-world reported gas mileage from owners on forums dedicated to Ford Econoline vans or camper van owners. Looking at resources like the EPA's FuelEconomy.gov (if applicable to the vehicle's year) might give you a baseline for the chassis, but the added weight and aerodynamics of the camper conversion will considerably lower the actual mileage.
