I don't know how you've tested the cruise, but some important information I learned after I purchased my '99 RSV.......
The cruise only functions in 4th and 5th gears, and then only between 45-80 (going from memory here). If you're testing the cruise in another gear or at lower speeds, it will not work.
Also, when setting the cruise, it typically drops off a little to start with and needs to be adjusted a little afterwards.
Lastly, if the cruise is working to hard (like in a strong headwind) if it drops off too much, it will stop working.
Good Luck!
Dave