Clean the walls – My recommendation for this would be TSP. It is a cleaner and degreaser made to use before painting, does an amazing job, and is dirt cheap. After cleaning, rinse the walls with water and let dry completely.
Prime the walls– If you want to use an oil-based primer, my favorite is Zinsser Cover Stain (Kilz is also a great choice). Oil-based works best (it’s the only thing that worked in the cabinets) but the clean up is a pain and it smells terrible, especially in a small space like an RV.