Wipe It Off
It is always a good practice to check to see if something is written on your trailer. You never know 
what people may write in the dirt on your trailer. It may be something like “Joe was here.”. It may be swear words. Wear your gloves. Use them to wipe of ANY writing on the back of the trailer.
Pulling a company trailer, I am representing the company. Even a one truck, one trailer company represents themselves. The equipment is seen by the general public and your customers. You wouldn't want to risk losing a customer, because some idiot scrawled something on your trailer.
This year's election seems more contentious than any in my memory. Some people may think that you're an idiot if you vote for that candidate, or you're a moron if you vote for the other one. Being politically active is fine. I have my opinions. You probably have yours. I am OK, if you disagree with me. I hope that you are OK, if I disagree with you. It is politics. I try to keep it away from my business. You won't see me promoting a candidate on Freightliner's website, Paper Transport's trailer, or my truck. I won't wear a vote for T shirt. It just isn't good business.
Unfortunately some people will get offended by politics. If you have a customer who feels strongly about a candidate, promoting the wrong one could lose you that customer. We work in a monopolistic 
competitive environment. There might not be much difference between the service, and price, that I provide my customer and another company might. Believe me, other trucking companies are knocking on your customer's door.
It seems these days people are looking to get angry. Everyday we see idiots on the road. People get angry. Could someone spark a road rage incident because they don't like what is written on the back of your trailer? Why take the chance? There are too many nuts on the road who see big trucks as a target. Anything can spark some of these people. It isn't worth it.
Before you drive your truck, you do a pre trip inspection. For me, that has always included checking my trailer to see if anything is written on it. If there is, I just wipe it off. It is just routine for me. These days it has taken on a little more importance.