Actually, I would not do as Lebron has suggested (sorry Lebron). It is better that the temperature just stay consistent than you change it constantly, especially as you won't be able to do this 24/7 until you get a heater.
Instead, wrap the tank in a towel or blanket and keep it high up in an enclosed space like a cupboard. Heat will rise and keep it nice and toasty. Alternative, set your heating to a constant 80-82F. That should keep the water above 75, at least.
Before you leave on your trip, do a 100% water change, and then do another when you get back. :)