Bob, you are correct. Your truck does not show any rust because all the sheet metal is galvanized. Hence the "rust through warranty from GM". The frame of your truck has been coated with sticky type underseal. All the HL frames say they are powder coated?? I have never ever seen a bare metal frame on their lot. Just how long does a coach and frame need to set on a lot covered with salt to get rusty. The snow covered roads I have been on, have both salt and sand on them. So driving on them would be like sandblasting them with salt. Like I stated in an earlier post. If HL wants to charge me for adding a good undercoating on a new coach....then I would pay it. OR, look under the coach you are buying, if off the lot and not ordering it, and see if it is rusty. If so , order one built in the summer time when the snow and salt is gone.
Anyway, sorry you have a rust issue.