I think its a great idea. I've been thinking about doing the same myself.
I don't advertise on my vehicles now and when i use to i never got any calls from someone saying "hey i saw your truck"
Mobile billboards is big business. In some cities you can make as much as $500.00 a week just driving a truck around with a large billboard on the back. So if you sit and think about it, you are already driving around as it is, so why not get paid during travel times. You could ask your equipment dealer if they would be interested or other companies you use.