G'day John,
Mrs T & I recently spent a month in Tassie, loosing two weeks due to a bingle. However, you could easily spend three months or more there. So much to see & do. You'll want a heater though. We had ours on most nights (10 Jan-11 Feb).
You could give the Kimberley & Cape York a reasonable go in three months. If the Tanami is open, Adelaide, Alice, Halls Ck, Fitzroy Crossing, Broome, Cape Leveque, GRR including Mitchell Plateau, Bungles, Halls Ck, Top Springs, Dunmarra, Cloncurry, Normanton, Mareeba (take a side trip to Kuranda & camp at Chateau TOPNDR), Cape York, home.
Take 6 days to get to Broome. 30 days in Kimberley (easily possible to spend more), 6 days to Mareeba from Halls Ck, 30 days Cape York (don't miss Cooktown), 6 days to Adelaide. Leaves you at least 12 days for the unexpected.
ps, I towed a Kimberley Kamper across the Simopson in '08 with a 3 litre 'Trol (manual), so don't be put off doing that.