I need to seal cracks in front top of trailer

Here is an idea that "might" work. Sand the entire front cap smooth, then apply a skim coat of "bondo" ( bodyfiller, you can buy it at any auto parts house ), then sand that smooth and paint it. How "good" of a paint job you apply would be determined by how perfect it needs to look ( or how, "less than perfect you would accept" ). You could do it all in an afternoon.
This is going to sound funny, but honestly, you could likely get a coat of paint on there that would be acceptable with a roller.