Dan,
Of course the most cost effective way is to take them and have them tested, and only replace them if they are bad. However, if your chassis batteries are 5 years old, in most cases, you are on borrowed time. So it might be advantageous to replace them this winter if you are not moving much like most of us, and you have the time and access to batteries at a reasonable price instead of waiting until you are on the road, pressed for time and having to pay an exorbitant price for the only battery in a small town.
Gerald