Since I'm the one who originally started this thread, I guess I better share as to how the cruise control issue was settled.
Since I'm a "newbie" to motorcycle riding I didn't dare to attempt anything mechanical on my own so I took my RSTD to the shop where it was found that a cracked vacuum actuator was the initial culprit. Once replaced the cruise control continued to not set, even when riding at the manufacturer's recommended speed of 50+ in 5th gear. Further investigation found that the engine was lacking insufficient power to feed the vacuum actuator. Subsequently all four carb float heights were adjusted following the replacement of 4 needle valve sets.
Once out of the shop, the difference on the road was very noticeable and the cruise control works like a "charm". I hope my experience benefits other 2005 RSTD owners.