I have to throw in a bit here...
I buy all my metal from AirParts Inc
. Their service and pricing cannot be beat in my opinion. Order before 12 and it goes out that day. To the East Coast it is second day service for me. Whatever I purchase, I always have it coated with a PVC film to minimize the scratching. It can be peeled off after you do your fabricating and is worth the extra cost. For interiors I use 5052 H38 in .040. I have found the extra thickness also worth the extra few dollars. I find .025 to be worthless. It buckles and bows way too easily. For anything exterior it is 2024. For skins, belly and patches I use 2024 T3 .032 in all cases. For exterior items that require bending you cannot use T3, it snaps on the bend. I use 2024 T0 in the .040 thickness for that. Eyebrows above the door are a prime example of where this comes into place. I also use it for new bumper compartments since I totally build them different than they were originally so no water can go under the floor.
Now for the part some will frown on and not appreciate me saying; Bare aluminum, of any alloy on the interior is a fools errand. I have done it numerous times on clients project and even in my own. It is a serious maintenance nightmare! It looks good when you put it in and then it goes all to heck. Every person coming in to see your trailer will, without a thought, reach out and touch it. When they do, the oils in their hands leave a finger print that will never go away. If you use it in the bathroom and it is within 4 feet of the toilet, any man using the bathroom will leave little white spots all over it no matter how careful he is. Interior aluminum is meant to be painted. That is why Wally used zolatone on the interior. He was a smart dude unlike the later Presidents who thought of the trailers as just a product.