I live in Beverly....next town over. There are very few places to park an Rv in Salem. As a matter of fact, parking of any kind is tough. The best solution would be to find a place on the train line (MBTA)and use that. The Salem stop is very near downtown, and shuttle busses and tour "trollies" are the way to go. I would check out the "Destination Salem" website for detailed information. Halloween night is awesome, if you like big crowds, and it's very safe. The Mayors office has done a good job keeping it family friendly with ample police, both local and state, plenty of security all around. With the Marathon issue this spring, I expect security to be even higher. I have 2 kids in College, and I have no worries about their security on Halloween Night. As for the rest of the coming weeks, it's a great time. Be sure to check out the Ghost Tours.
Good luck....and have fun !!
Alan