Hello Rob,
You don't seem to be getting the answer to your question do you, so let me try.
In principle yes you could use the the plug but in practice it may not very well or it may not work at all.
Principally it depend if your caravan has a habitation relay or not. If you have one, then the plug idea wont work, because the Habitation relay will disconnect the charging pin in the front plug.
If your caravan is 'olde' enough not to have the HR, then yes it would work, but because of the extra length of the wires, the solar charge rate will not be as effective. Most 12V solar panel installations try to keep wire lengths to a minimum.
Many battery boxes have either a cable entry point at the bottom or some drainage slots. As a suggestion, you can get water proof connectors with a short wire suitable for solar panels, which you could more or less permanently attach to the battery and leave inside the battery box. When you come to connect the panel, bring the connector outside the battery box lay it in one of the channels to the outside and then connect the panel.
Hope this helps