Is there a way with RoboHelp to add an image or text that would identify some content that is published as being 'new' and would then automatically disappear in a set amount of time, like 30 days? With the amount of content I continually add, this would be a handy feature and users could quickly notice any new content.
Unfortunately there is not such a feature. If you would like to suggest it (which I highly recommend) try the link below.
When I was in the corporate world with hundreds of users depending on updates, I handled these by creating a "What's new/What's different" page. I then created 24 different See Also keywords. NewJan, NewFeb, NewMar... RevJan, RevFeb, RevMar... and just created links on the What's new and different page to the keywords.
This self managed things. It provided me a way to easily maintain a rolling 12 month window of updates. All I had to do was remember to associate the New and Changed topics with the appropriate SeeAlso keyword!