I think one needs to do a cost benefit analysis on your theory. I am not the expert on building coaches but I have a solid, relative education and professional background manufacturing.
1.First, one would need to engineer the structure for steel and then re-engineer it again for aluminum. Structurally, you may need to increase the material thickness and size to make it work with aluminum. This will likely be the first rude awaking to fabricating large structures with aluminum. It can be very expensive to do when compared to steel. This is especially true in the “over-building frame of mind” that we tend to work in when we don’t have access to proper stress analysis engineering and design services to structurally optimize the design. We throw material at the concerns to insure there will be no failures. Otherwise things break and bend where we least expect them to do so.
2.The next reality is that structural aluminum is more difficult to weld than steel. It’s easy to stick together but much more difficult to control the twisting and warping that occurs when you apply too much heat or weld assemblies in the wrong sequence. It can be done but requires more skill and is therefore more expensive. And it is even more expensive if you don’t have the skill since you ruin lots of good material. There are lots of manufacturers making good aluminum frames for vehicles but it is a different expertise and a more complicated process than what most of us are willing to pay for.
3.Another concern is welding aluminum plate for the outer skin. This is not really an option. The heat would destroy any reasonable thickness of aluminum skin and any skin thick enough to weld would be crazy expensive and getting pretty heavy. That is the reason they use bonding (tape and glue) or mechanical fastening (rivets or screws) to attach thinner sheets of aluminum.
I briefly looked into building my box top from aluminum and quickly realized that there was no benefit to the added cost of doing a steel floor structure and adding an aluminum box (don’t forget that steel and aluminum ideally should not touch each other).
The weight savings just didn’t pay off. On a 35Klb rig, saving 1,000lbs just doesn’t buy you much at 10 to 20K miles per year. Now if your driving a 100K mi per year and you can get paid for an extra 1,000 lbs of freight every where you go, that is a different story all together. And if you’re building an airplane, it’s really worth talking about. However, for a one-off coach with minimal engineering resources available I don’t think I could make it work. If we could afford to engineer it and build it with aluminum, we probably wouldn’t be building them ourselves and on used truck frames. But then the fuel savings might not be an issue either.
Good luck on what ever you decide…
T2000 Complete (but never finished)