Don't put the cart before the horse my friend. Re-look at exactly it is what you want to accomplish. If all you want is to increase business/cash flow etc, etc,.. You'll be pissing away money on an enclosed trailer. The very best advertising is and always will be word of mouth. So before you go and drop $3k, maybe you need to increase the quality of your work if need be and/or network your existing clients better. I've gone down this road and a wrapped enclosed trailer will increase and/or bring in new clients 6-8% at best. People simply aren't going to grab a pencil and paper while they see you on the road and write down your contact info while they are driving. And at best, while your parked in front of a clients house you'll get maybe 10-20 eyeballs at most on your info depending on traffic flow, population, etc....etc....
Now that being said, and enclosed trailer offers added security, some advertising, (your name can get stuck in folks head, if they see it enough), and yes, it offers professionalism. But you pay for it. An open trailer is cheaper, and much faster to load/unload.
For me personally, the best return on my money has been the small wire real estate type signs. They are there 24/7, and get much more eyeballs than anything else. Work out a deal such as 10% off or a free service every 3 months or something like that for letting your place it. People normally chunk door hangers, rarely ever see your trailer if they work during the day unless it's a weekend, and who reads a newspaper anymore?
For me, custom tee shirts, business cards, yard signs, and quality work will beat anything else hands down. Everything else is a waste of money, in my case.
But if your stuck on a enclosed trailer afterall, contact me. I have a 2008 16' Featherlite I'll sell ya. And it won't be cheap. Good Luck.