The actual process of shortening the roller tube is no big deal, but the calculations for getting an exact fit to your trailer must be precise!!
The first measurement you need is the width for the new trailer. Look directly down from both ends of the awning rail at the ribs below and measure ACCURATELY from rivet center to rivet center. This is the center line width for the new awning.
Lay the awning out on the ground (still rolled up) and measure from the centers of the support arms that attach into the roller tube. Deduct the rivet-to-rivet dimension and you have amount that the roller tube needs to be shortened.
Then unroll the awning, remove the fabric, springs inside the roller tube and the arms. Use a chop saw with a carbide blade to cut the roller to it's new length. You will then have to get the fabric (if it is still useable) trimmed by the same amount you took off the roller.
When it's back from the canvas shop, re-assemble everything, re-wind the springs and bolt that sucker to your trailer.
Actually, cutting is the easy part. It's getting everything set for the re-assembly that takes the time. If you've never installed an awning, be ready for a full day....with 1-2 helpers!