[Image of engine diagram]
The harness that came up through the floor by the dash is likely the ignition harness. It connects the ignition switch to the ignition coil, and it also supplies power to the starter solenoid and other ignition components. The harness should be routed through the firewall and connected to the ignition switch.
Here is a diagram of the ignition harness for the 1976 GMC motorhome 26ft palm beach:
[Image of ignition harness diagram]
As you can see, the ignition harness is a complex network of wires. If you are not comfortable working with electrical systems, it is best to have a qualified mechanic repair your vehicle.