I just ordered in a big roll of roofing aluminum from All-Rite. I've known about this product for quite a while but had never seen it till it came in. This may be an answer for folks building over 48" wide. Its .025, and after a trip across the country it didn't look bad. This stuff is an approved RV roofing material. With backing it should be just fine. All-Rite will sell to you.
Of course if you can find wide Aluminum Locally your probably better off.
http://www.all-rite.com/
Happy Trails
Len