My trailer has solid sheet metal sides and have the length of it lettered (vinyl) with business name down the entire length of the sides along with phone number. Can't tell you how many calls I get from that. It's one of the very best forms of advertising I use.
It is also a sales tool in that I tell people, "look, my reputation is only as good as the quality of my work. My truck and trailer, plastered with our name and phone number, is parked right in front of your house. There is no secret as to who is maintaining your landscape. Don't you think I am going to do everything I can do to make your place look the best it possibly can, at the same time protecting our good name and reputation?"
My trailer is lettered-its an open trailer. Get this, I run my business out of my home and when I went to get my business license they told me that my trailer could not be lettered but my truck could. Does that sound wierd. But at this time I'm not changing my trailer. I will have to keep it out of eye shot for awhile but what ever works.