Electrics ?

Morning all,

this may seem a silly question. We are relatively new to caravanning and have booked a rally in August, the only problem is there are no electrics on site. 

1. How long would you expect the 12volt system to last without charge and:

2. how long when using a 240 volt inverter on the 12volt system. My wife needs to use some 240v medical equipment twice a day.

I suspect this all sounds very basic, but i was always told if you dont know ask. :)

Thanks

Mike