You can be on 495 in minutes and can zip up to Salem.
Sometimes. Best case it's a 45 minute run. That's no traffic. No matter what you have to cross over to get to I95. Salisbury is closer by about 10-15 minutes (no traffic) but it's a more direct route.
Its a little west however we make Cambridge in 30 minutes.
At what rate of speed? Best case, no traffic, straight down Rt. 2 it's a 40 minute drive.
OP, I'd look into Winter Island. There may be another campground that will allow you to dump your tanks and some towns also allow it at their sewage treatment facilities. There's not really any such thing as "not too much traffic" in eastern MA.