Joann,
I would think that the entire belly of your trailer should be covered with
soft aluminum sheetmetal. This is very pliable sheetmetal -- I think we
used to refer to it a '0' treated aluminum. That's pronounced as "oh", but
it's shown as a zero. In the case of some aluminum alloys like 2024 the
belly skin would just be 0 or almost pure aluminum sheetmetal. The alloys
make aluminum stronger but you can heat treat 0 aluminum after it's formed
to be very strong.
Tom