1999 Plymouth Voyager car overheating

I just got my van from a used car dealership 2 months ago, and each week I am having to put in a whole bottle of antifreeze at it is always overheating. I dont go anywhere but here in our small town, and I never notice a spill or leak coming out of my radiator. Is it normal for this type of vehicle to "drink up" the antifreeze and the reserve like this? I bought my van with out a warranty, but still making payments on it, should my dealership check it out? I dont want to have to spend $ at a car repair shop just to diagnose it.