If your Southwind is the same vintage as mine, you probably have a "RV Custom Products" Battery Control Center. If you can hear the solenoids connecting and disconnecting when you press the switch, there are two possibilities:
1) In the bottom left side of the BCC are two small circuit breakers, press the reset buttons on these to reset them.
2) There are two battery disconnect solenoids inside. These are magnet based, i.e. in the "USE" position, a strong magnet holds the contacts in. On mine, the contacts were intermittent and the center plunger would occasionally just drop out by itself.
I disassembled the solenoid and cleaned the center plunger and the sleeve it slides in to. I also compressed the outer spring a little so it put a little less pressure in the contacts when the solenoid was pulled in.
Note that the contact are part of the connector bolts. The contacts can get twisted a little when tightening up the connector nuts. Realign these so the large contact washer makes good contact.
Scroll down to the bottom of this link, there are a lot of photos on how I disassembled by BCC. WARNING - disconnect the coach and chassis batteries, and make sure shore power is disconnected before disassembling the BCC