I have thought of some of these things lately, and have been in sales and marketing most of my life.
My first idea would be to get some type of very professional vinyl graphics for my RV, showing the type of services offered. You may however, get bothered at all hours of the day or night at your campsite. ( kind of like when you were employed full time as a plumber)
If you don't like that, you would need a toad, or just your towing vehicle to be wrapped or advertised, and then later you could cover it, or hide it from site.
If you are traveling about, some type of social media contact page may help ( I am not really sure about Facebook) letting people know where you might be in advance.
If the camping site that you are staying ( for a while) may have a newsletter or website with advertising available, that you could contact about an ad offering services.
I have other ideas that I will post another time. Good luck!