There is two types of grounding involved with the antenna:
Ground Plane: Most antennas require a grounded flat plane under the antenna for it to work properly. This is different from an electrical system ground. The outer shield of the coaxial cable would normally be connected to the vehicle body ground, or in this case, the trailer ground. However, magnetic mount antennas use a capacitive coupling that works fairly well. So, stick it on top of your trailer on the metal roof in such a way it won't blow/fall off and you should be OK.
Lightning/static ground: This is usually a good idea, but considering the circumstances, I agree with JWT. In a commercial radio site the coaxial cable shield would be grounded at several points and a lightning arrestor would also be installed at the point the cable enters the structure. In your case, unless the trailer is alone at the top of a high hill, you're probably pretty safe. It wouldn't hurt to ground the outer shield of the coax, but it's up to you.
Your trailer frame/body should be grounded locally, usually at the point where the electric utility service is attached. I'd suspect you'd have bigger issues if that wasn't the case.
So, stick it up on top of the trailer and give it a go.
