No
The comma is not required. As @EdwinAshworth pointed out above, some grammar guides will advise it in order to distinguish a command to greet someone from the greeting itself. Really, it's perfectly clear which of the two senses is intended from context.
On the other hand, it isn't impermissible either, just because it's a salutation.
In practice, what you're going to see is people who write it Welcome Campers as a headline or who say it quickly and others who write it Welcome, Campers[!] who consider it a sentence and will speak it with a pause between the two words.
Edit: I thought we were just talking about the sign. In an email, I'd view it as a sentence and include the comma. Your examples are backwards: the version without the comma (even heading an email) shouldn't have any punctuation at the end. The version with the comma (even on the sign) should be punctuated.