What year is your Southwind? I assume its a F53 chassis.
This diagram is a 2001 F53, yours may be identical.. If you have an ohmmeter, you can unplug the instrument cluster and check the resistance between pin 10 (Yellow/White wire) and ground. If you have a variable resistor (or an old sender), you could even connect it and see if the gage responds.
As for the sender. I changed my fuel pump last summer, This is how I did it (with photos):
F53 Replace Fuel Pump – 1999 Southwind 35S