Oops, guess I missed the Yamaha Roadmaster part!
It sounds like you're looking for PPM (pules per mile). Looking at the instructions it sounds like there is one magnet on the drive shaft which means for every rotation of the drive shaft there is one pulse sent to the cruise control module.
Your diff ratio should be 4.56:1. I'm assuming your tire size is 8R 19.5 so the revs per mile for that tire are 621/revs per mile. 4.56 x 621 = 2,831 drive shaft turns per mile or PPM = 2,831.
The dip switch settings provide for 2,000 or 4,000, 5,000 or 8,000 which aren't really close to what your actual PPM is.
Looking at the CCS-100 manual it says you can also use the tach signal as a speed reference. I've actually used an aftermarket crusie control that used the tach signal and it worked just fine with one exception. If your transmission craps out then the cruise control will start to rev the engine as it tries to keep your speed up. Net result for me was an over revved engine after the transmission blew up. So I guess I wouldn't recommend using the tach as a speed signal!
Another choice is to use a signal generator
. This is what the normal Classic motorhome used as a speed reference for the cruise control. GM used speedometers that were expecting 1,000 revs per mile from the speedometer cable. So installing this signal generator creates 8,000 revs per mile that can be sent to the cruise control module.