Have you had a look in Yellow Pages for metal stockists? Have a ring round telling them what you are looking for and what you want to use it for. They may be able to help. It's worth a try. They would probably cut it to size for you if you give them the dimensions. Remember, measure twice, cut once!
The snag might be getting aluminium sheet in the thin guage and sizes you would need for the motorhome. It is difficult to stock because it gets damaged very easily and metal stockists tend to shy away.