We actually used windshield wiper fluid to pour into the tanks because it was 89 cents a gallon. Obviously, this is just for going right in the tanks as it's potentially toxic. The level of aanti-freeze protection is a bit less, but to be honest, we used it in sub-zero temps and it kept everything from freezing by the valve (which in our rig is not a heated area).
The Green Machine -- 2000 Mountain High Coachworks Summit (Spartan chassis / Cummins ISC)
...and F-Troop: Fearghus, Fiona, and Frankie (Cairn Terriers)