I'm not familiar with any in the modbury area at all. Not to say they don't exist. There is a park in windsor gardens, probaby a 15 drive to modbury.
http://www.windsorgardenscaravanpark.com.au/
A slight higher class park would be http://www.levipark.com.au/ but that is a bit further away. There may be closer ones near elizabeth but not sure I'd trust any camping areas in those neighbourhoods.
I have stayed at the Windsor Gardens Caravan Park last month @ $28.00 per night (solo), very clean and comfortable, some drive thru sites and population of regulars and a few permanents.
Lovely walking access to river where one catch a good look at the O'bahn Bus system zooming thru to and from the city.
Location is under the flight path but one soon becomes used to that, or if your anything like myself who rarely sees planes flying over will find it interesting.
