Ours had a little rust on it the day we bought it new. It's turning two years old now and there are some area's on the sheet metal that definitely need some touch up. I plan on attacking that this summer with a rust converter ( spraying those areas I can reach) and then rust preventative paint.
The chassis frame, axles and mounting brackets should be fine. If I can spray those with the RP I will. I don't believe there are too many MH that don't have some rust someplace. Like others indicated surface rust is acceptable, but if it's eating holes in the metal, that would raise flags.