Cloyd,
The sender (tank unit) should have two wires, a positive (signal wire) from the dash gauge, and a ground wire assuming that the sender does not depend on the tank as the ground. The dash gauge should have three wires, a positive to the battery, a signal wire to the sender (tank unit), and a wire to the gauge light. In some cases there may even be a fourth wire to the dash gauge to ground the light if the gauge is not case grounded.
Most of my experience has been on SMC era coaches and the automotive industry, and that is how they were wired. The only three wire senders that I am familiar with have an in-tank fuel pump and the third wire was the pump power supply. However, since your coach does not have an in-tank fuel pump, I do not know why you would have a third wire unless Monaco wired the fuel gauge backwards and grounded the circuit at the dash gauge. You will have to check the circuit to determine if this is your case.
Normally a fuel gauge is diagnosed with a variable resistor. The dash gauge should read full with the maximum resistance for the system and that can be from about 30 to 250 ohms depending on the dash gauge design, and empty at 0 to 10 ohms depending on design. A variable resistor is required when diagnosing an operational but inaccurate fuel gauge. For a problem like you expressed, I normally just disconnect the sender wire at the tank unit and see if the dash gauge goes to max full reading and ground it to see if the dash gauge goes to max empty reading. That will tell you that the gauge wiring is OK, and if the sender ground is good, your issue is not wiring related. Then you need to take an ohm meter and check the resistance in the sender (tank unit). If the sender shows an open circuit, it is bad an your dash gauge will be pegged to max full. Since you did not say which way your gauge was pegged, I am assuming to max full, but if it is at max empty, that would require a shorted sender or wire. If this did not make sense to you, just call me, and I will walk you through the process 713-254-4156
Gerald