1. There are only small variations. Some do have the rear wrap around windows but you lose the closet. The diesel pushers have the same floorplan but there is no doghouse up front and the engine is under the bed. I think they all have the north-south beds.
No wrap arounds.
2. The Chevy P-30 chassis and 454 engine are solid. There are hundreds of thousands running around as bread trucks and SOB motorhomes. Engines are engines and some folks take better care than others. Also, they need to be run. Let it sit for too long and you're looking at a replacement. Always buy a motorhomes that's been used or at least run regularly. Also, you can get a new crate GM 454 rather than a rebuild for about the same price. That's nice to know.
3. Fuel injected was added sometime in the early 90's.
4-6. I've never had a 360 but I hear the mileage is better. It is probably better due to the Fuel Injection and the 4-6 speed transmissions. The older classics have 3 speed transmissions geared for 55 mph. Push 65-70 and mileage goes way down and engine temps go up. I'd be skeptical of someone claiming 14+ mpg. 10-12 is my guess. You might want to ask that question at www.airforums.com
. There are several 360 gasser owners there that could give you real life data.
7. Look for road-side and rear awnings. That's the dead give-away for an upgraded unit. As a former Airstreamer, you know the vaule of the road side awnings. These units are pretty loaded all around.