Welcome Aboard! I see you are from Australia. I am not sure how the Australian campers are wired up.
As for water tightness, I have never seen water being an issue. On the USA trailers, if it is a twist lock style there is a seal at the TT side. I just keep the male end off the ground and I have never had any issues.
Generally speaking, most RVs have a lot of parasite power drains on them when just sitting. So you need to disconnect the battery or keep them plugged in. Almost everyone I know use a small "dog bone" adaptor to keep their RV plugged into a 15 amp outlet in driveway. Again I am not familiar with the Australian wire configuration. Hopefully someone will chime in with more specific answers.
Cheers,