I agree with try output speed sensor about $20 for aftermarket to try. Start with the basics bad connections,fuses ,damaged wires , grounds you may get lucky.
I'm in same boat Speedo and cruise went out. But mine is electronic either Cummins or Allison same as into 2000s electronic injection and cruise. I asked here who's it is not much help.
95 Monaco Crown Royale
M11 400hp, 4060 trans.
Aquahot, Generac Guardian7.5k