Check your battery connections, and also have the batteries checked for weak cells. If you are moving slow, and you are putting a heavy demand on the electrical system, the alternator will not be able to produce enough electrical current to keep the batteries charged. If you have a battery switch, I suggest using only one battery, so should you deplete all the current in that battery, you can shut it off and get the car started on the other battery. Also carry a good quality heavy duty jumper cables with you, because you are going to need them one day. I will bet that your car has a very large siren on it, like a Federal C6 or a Q? They are definitely energy hogs, unlike a Super Chief.