I had stains on my ceiling, running front to rear. Took it to the factory. At first they thought they would need to replace the ceiling!
They tried several cleaning solutions and got one to work. They would not or could not tell me what they used. It seems it was a home made brew by one of the Amish worker's family. Everything is fine with the ceiling.
First consider determining if there is or is not a continuing source for the stain. It may not be a leak, but a reaction to the unit being cleaned for delivery to you. Call Newmar at 800-343-7357. Ask for technical support. They are very helpful.