OP was not in business with his neighbor. He was not an employee of his neighbor. He helping out his illiterate neighbor and there is nothing that OP posted that would indicate that he had any knowledge the neighbor was operating a non-licensed business.
However, I would like to know who handled the title transfers. The neighbor was obtaining these RVs. Was he buying them or acting as an agent for the owners? Did he purchase them, title them in his own name and then sell them for profit? Was he jumping titles? And what did OP know of all this?
If the neighbor was only working as an agent for owners that wanted to sell their RVs, he was not a dealer and I don't see him as operating an illegal business.