I admire your choices and I hope it will work out well for you!
In regards to fuel, I too would echo that you stay away as much as possible from waste vegetable oil and large percentages of bio-diesel.
Twenty years ago waste vegetable oil was a waste product users paid people to take off of their hands. If someone showed up at their back door and said they would take their WVO off their hands they would give it away. But two problems for bus owners have come up that makes WVO a non-starter for bus owners.
First and foremost is the number of gallons you will need to get anywhere. It is one thing to walk up and ask for 10-gallons of WVO for your diesel powered Mercedes-Benz that gets 30+ MPG. It is a totally different issue of finding enough WVO to fill a 110-gallon on a bus that gets less than 10 MPG.
The second real problem with WVO is almost no one will let you take the WVO away for free any longer. Most vegetable oil users do not purchase the oil and pay to get rid of it. Most vegetable oil users lease the oil from a company who delivers the fresh oil and picks up the waste oil for about the same price as what vegetable oil used to cost. That company then takes it back to refine the WVO to be sold as a bio-fuel that can be blended into regular dino diesel. Or uses the WVO in their own trucks.
As it has been noted before, propane is a good fuel but has some built in problems with the biggest one finding a fuel source while out on the road. Gasoline engines that were converted to run on propane are notorious for their lack of power. The newer buses that have engines that were designed and built specifically to run on propane do a lot better but they still have a lack of range and difficulty in finding fueling stations. The buses that run on compressed natural gas are some of the best for running clean but they define hard to find fueling stations away from home.
The only buses that used aluminum in the construction of their buses was Crown and Gillig made some of their buses with aluminum skins. The difference in weight was 6,000 to 10,000 pounds depending on the size of the bus. But even if their skin was aluminum their framing was all steel.
If you are looking to purchase a lighter weight bus to save on weight and get better fuel mileage there are some better ways in which to achieve better fuel mileage.
First, even though it may sound counter intuitive, find a bus with the greatest amount of HP. A large HP engine that doesn't have to work very hard to go highway speeds will use less fuel than a small HP engine that is firewalled every time you get out on the highway.
Second, reducing the drag can make a huge difference in fuel use. I have owned two different Avion travel trailers that were close enough in model years to have the same frontal area. One was a 26' LeGrande that weighed in at about 4,000 lbs. The other weighs in at just over 7,000 lbs. Both have roof top A/C units, roof top tv antennas, and full length awnings on the curb side. I have towed both trailers with two different tow vehicles enough miles to know that I use virtually the same amount of fuel to tow either trailer. How does that equate to reducing drag? What it tells me is weight has little to do with fuel use but frontal area and overall drag makes a big difference in fuel use. So how does that make a difference for you--the lower the roof, the less on the roof, and the less hung on the sides of your bus the less fuel you will use. It takes a tremendous amount of HP and torque to get your bus up to speed. Increasing your drag will require greater amounts of HP and torque to get you up to speed and to keep going at that speed.
Probably the best bus out there that would meet your basic parameters would be an IC RE with the DT530 with the 300 HP or 335 HP option or a Thomas or Blue Bird with the Cummins ISC or ISL engine option with 300+ HP. Any of bus equipped with those engines are more than capable of maintaining highway speeds and will use less fuel to do it than a bus with a smaller engine. There were some buses built with Cat engines with 285+ HP out there but I would stay away from them mostly because they don't call it Caterpillar gold for nothing.
One other thing to think about the big HP buses--most were spe'c'ed and purchased to be trip buses. They generally had a lot of added cost options on them. Among the popular options for trip buses in addition to the big HP engines were pass through under the floor luggage compartments, highway gearing, tinted glass, rear air ride suspension, 12" windows with 76"-78" headroom, vandal locks on all doors, air ride driver's seat, tilt/telescope steering column and on newer buses adjustable pedals, and white roofs.
As far as livable space is concerned, there are several builds chronicled here that include slide outs and roof raises that add more floor space and cubic volume for more headroom or a "second story" loft for sleeping.
With anything added it does add weight. And while weight alone does not make that much difference in fuel mileage, it is true that a bus that weighs 20K lbs. is going to use less fuel than one that weighs 30K lbs.
Good luck and happy trails.