I had an 88 Tristar that had issues with the starboard speedo. There is a small hose that attaches to the speedo pitot on the stearn underneath the platform. Make sure that hose is connected. I eventually replaced the entire pitot. You can order them from any marine store like Bartswatersports.com. After that, the speedo worked 90% of the time. If we were underway and hit a large wake, it would sometimes drop to zero and then work again once we slowed. I am not sure about the RPM gauge.
Hope this helps!