the inverter i run has a remote control panel, the link is here for it
http://www.ebay.com.au/itm/PROJECTA...=AU_Car_Parts_Accessories&hash=item43b64e8611
the inverter is hidden out of sight, the power wire running to the power points were cut and a changeover relay was inserted in its place, also with the output from the inverter going to the relay.
the relay has a 240 volt coil on it. the coil is connected to the caravan mains power circuit.
the normally open contacts are connected to the caravan power circuit.
the normally closed contacts are connected to the mains output of the inverter.
the common connections of the relay should be connected to the power sockets of the van.
the relay should be at least 15 amps 240 volt contacts, and must be double poll contacts, as you have to switch both nuetural and active wires.
when there is no power the relay has no power and the inverter runs through the relay and powers the power points.
when there is power the relay is powered and the inverter power is cut and the main power runs through the relay to the power points.
simple design and works seamlessly. you can have the inverter on all the time, and if power is cut, you would not even know it.
a relay and base on ebay would be on this link
http://www.ebay.com.au/itm/JQX-13F-...Electrical_Test_Equipment&hash=item54047ec5eb