What is the best way to apply the outer skin and roof on metal studs? I am using steel studs and aluminum skin.

I would suggest Sikaflex 252 for your walls. The steel (& alum) needs to be completely cleaned & void of any oil or crud.
This is the same product used in the manufacture of commercial truck bodies.
Once completely cured (aprox 24 hours) it is impossible to separate the bond without destroying the aluminum skin.
The best way (that I know of) to attach the skin, is to install a temporary ledge at the botton of the structure to set the alum panels on. This will hold them in position while you're working,... then use some straight 2X4's over the outside of the skin as a clamping bars. Attach 4 or 5 clamps spaced out along the length of the 2X4's & snug the clamps evenly.
The steel tubing (studs) in the middle of each panel should be ok without clamps,...just use a piece of 2X4 & a mallet to gently "seat" the panel to the studs.
Depending on temp & humidity, the Sikaflex will cure to the point of holding the skins in a few hours.
Don't get in a hurry doing the complete job,...let the bond on each panel cure before moving on to the next one.
One source for Sikaflex is Austinhardware.com, It isn't cheap but it doesn't de-bond like 3M VHB has been known to do.
Again, if you go this rout,...the emphasis is on taking the necessary care in cleaning & careful installation.
You may want to consider a .030" or .040" aluminum one piece roof with 5/8" or 3/4"plywood underneath. You can bond the alum to the plywood with contact cement -or- just leave it loose. Your roof components will hold it down in the middle,...ft/rear & side rails will fasten it around the perimeter.

...I AGREE WITH BOB-He is totally correct on going slow and using Silkaflex....I can't stress enough in bonding after cleaning almost hospital clean[also use the primer]...I screwed one up big time-bonding is a slightly dusty environment and It cost me week of my time and getting my azz reamed to get it straightened out...it was a nightmare. I was going too fast and not double checking out what I was doing/not doing....although a smaller box type van it was a very profitable install for those portable billboards that you see driving around town....I may have broken even but I doubt it.....I had 4 ea. .062 aluminum painted sheets that I had to eat 48X120...gulp!....Now I no longer bond-I screw them on or I have it done by the trailer/coach body builder....even insurance losses are sent out....never-never-never will I try to cut corners....And that is Written is Stone........geofkaye

Thanks for the information. The wall skin will be .050 and it's 49 x 120. The roof will be aluminum treadbrite 3/16 thick and continuosly welded.