1. Yes a local dealer "can" take care of warranty issues. But "will" he is a totally separate question. He may or may not, you should ask him. Many won't or will only do it if he has nothing else to work on that day.
2. There is always a length limit. Some it may be so astronomically high that it's not even a consideration. Some places the longer sites get reserved so fast that it's for all practical purposes it's impossible to get in. In general, a 19-foot TT will never have a problem getting a site (if the campground isn't booked up). The longer your TT, the less and less sites you will fit into (you can't put a 40-foot trailer on a site that is only 20-feet road to road). The longer you wait to reserve, the less and less likely you are to find a site to fit into.