The enerdrive website states (on one of the videos) that you need a sparky (as
@Drover has stated). If you are using the enerdrive unit with a transfer switch built in then the 240V into the van goes to enerdrive unit and then the enerdrive unit has another plug that goes to the caravan power points, aircon etc.
For the enerdrive unit, the power point on the unit is not used as part of this installation. It is an independent outlet.
You can choose to have the sparky wire in such a way to include the A/C or not, meaning that instead of having the outside input go to the inverter and then the inverter to everything else. The input to the inverter would be from the power point feed and then the output from the inverter to the power points.
I would also suggest that the 240V battery charger should be isolated from the inverter circuit so that it is not energised while running from the inverter (ie Battery to DC to AC to DC and back to battery does not charge batteries).
If you do not have a unit with an inbuilt transfer switch, then you need to get the sparky to wire in a manual (or automatic) one if you want to use the same power points.
There is a crude option (do not know if it is legal or what the requirements for isolation switches are), where the installation could be an outlet on the outside of the van powered by the inverter, which via an extension cord is plugged into the external 240V inlet. Issues here with 15 Amp compatibility. Again a sparky is required.
If I have not made it clear a sparky that deals with this, or is going to chase down the correct regulations is required, and you need to discuss what will and will not be able to operate with the inverter and size the inverter correctly (including cooling capacity - air flow).
Finally, as
@Drover has said, is option 1 actually use an appliance that is not 240V to achieve what you want or a generator (keep it 240V). As a minimum the currents involved when converting from 12V to 240V is a multiplier of 20. That is 15 Amp 240V will require 300 amps at 12V. This means that connections on the 12V side need to be capable of handling the currents and any small increase in resistance (bad joint / contact) will result in heat (0.0001 ohm resistance would generate 9W of heat with a voltage drop of .03V). For comparison a 12V - 25W globe has a resistance of 0.5 ohms and draws approximately 2 amps.