I painted my 1966 camper twice. I used enamel oil based house paint.
First, I thoroughly cleaned it with TSP. Back then you could get real TSP (tri sodium phosphate). It was fairly caustic. You had to wear rubber gloves. Now they have what is called TSP substitute.
After the surfaces were cleaned, I combined one quart of Penetrol to one gallon of paint. Penetrol is basically linseed oil.
I used a brush and a roller to apply the paint. It was a durable and shiny paint job that lasted years.
After the paint dried for a week, you could even wax the vehicle with car wax.