A couple of horses got to the side of my trailer over the fence.  There are several scratches down to the aluminum and part of the window frame.  What is the best way to fix this?  I have done a search for scratched paint but don't see anything that addresses what type of paint and/or primer to cover up scratches.  Any comments appreciated.

You can fix the scratches, but it's almost impossible to easily match the paint colour. There are a hundred shades of white, and the OEM sheen is not similar to a normal flat or glossy automotive paint. Go to any auto parts store and buy some rattle can spray primer and white paint, along with a small amount of some acetone or cleaner/degreaser.

Clean the bare metal, spray some primer on a piece of cardboard until a puddle forms. With an artist's brush, dip it into the puddle and apply the primer to the bare metal. When it drys, add a second coat. Then add the colour coats the same way until the scratch is filled. In all probability the colour will not be accurate.

If an exact match is your only goal, you can go to an automotive paint supplier that provides paint for body shops. Most can computer match your present paint colouring, and add the necessary flattening agents to match your present paint. They can mix a small batch and sell to you in a spray can. If there are extensive scratches, it may be necessary to repaint an entire panel.

.... Or you could leave the scratches as they are, it's not going to rust being that it's aluminum and it gives your trailer it's own unique character!!  Sometimes these battle scars are best left alone (unless it looks like something has taken a chisel to the paint) if the repairs have not been professionally done as Gard posted.