Why bother with fixing it?
Your foot can easily handle the task and have greater control of the speed which will result in better fuel mileage and much more stable speed since you can do something that cruise control cannot.. Anticipate the hills BEFORE you get there and lightly apply fuel BEFORE you start losing speed.
I tried cruise control once and noticed that when I came to any hill, it would wait until you lost a preset amount of speed then GUN the throttle to make up for the lost speed.. Might work OK with a lightly loaded car with bunches of excess HP and TQ, not so much with a heavy load like a RV that does not have excess HP and TQ for the load.
28+ yr old vehicle is pretty darned old, most parts have been taken off the books, off the shelf, off the market and often destroyed after a vehicle is 10 yrs old so you are 18+ yrs late to the party.
Troubleshooting can be a bear, sometimes design is changed midstream of the year or you might get real lucky have one that was built for one model year and nothing else fits or works. You really have to have a pretty good understanding of how your specific cruise works mechanically and electronically in order to troubleshoot without shotgunning parts at it.