you nailed it right on the head with your answer. better mileage, lower cost (tires, fuel...), better ride...
could I ask you a few questions re your conversion?
1. what is the approx length of the couches in the sleeper shown here? 6 feet, longer...?
2. is there any problems from having such a large opening between the truck and the conversion, like leaks or stuctural weakness...?
2a. is the joint a solid or flexible joint?
3. what percentage of conversions that you've seen do you think have a large opening like yours?
4. do you wish you had windows on the side of the sleeper above the couches similar to this or do you like the solid walls? http://truckconversion.net/photopost...php?photo=1320
5. do you couches have seatbelts?
6. have you ever thought about adding a removable center table in your sleeper between the couches so people can eat, write...?
7. did you order your conversion of was is allready built?