Thank you for persisting, Hikerdogs. I have learned something from you today.
At least some of the "illegal" characters (including the percent sign and the semicolon) are strangely permitted if you use a secure connection to post by changing the URL for the forums from http://www.rv.net/... to https://www.rv.net/.... You can put any (unicode) character in a post using an HTML entity, but usually that's more work than it's worth.
The "Preview Message" link will show the same error if you have an illegal symbol, but not lose what you're typing.