What is the best way to approach a company about advertising on my RV?

I want to sell advertising space on my RV. I live full time on the road, and travel the country. If I had money from people paying me to advertise on the RV, I would be going a lot more places, advertising a lot more. So what is the best approach at getting a company to advertise with me? Anyone have any experience with this?

Probably through the door. Coming through their window would be weird.