Unless you like camping i wouldnt bother sleeping in the back (Done it b4 and very cold). You also have to check if the back folds out flat as my patrol is impossible to sleep in the back!
I would suggest going to a nice B&B in Mansfield and then go to mount stirling (Or Buller if u really want to) for a day trip. It will cost you bugger all, has all the snow you need to play in and is a great drive from mansfield.
Or stay in melbourne and go to Mt Donna Buang or Lake mountain for the day. Great day trips