Dead Battery

I have a 1992 Chrysler Town & Country I have owned for 2 yrs., I have gone through 2 batteries in that time, the van is fully loaded (power everything), and has the 3.3 V6. I also have a 1991 Plymouth Voyager, Base model, and that took 9 yrs. to go through its battery. Anyone have any suggestions as to why it is doing this?
(tried the battery from the 91 in the 92 so nothing wrong mechanically)

Connect an amp meter in series between your disconnected positive battery cable and the positive battery post. Shouldn't see much more that 50 milliamps. If yes then a system is slowly draining your battery just enough to "deep cycle" it without giving you weak battery indications for months. Batteries don't like to be drained and recharged more then they are designed to be.

Start the car and make sure the voltage is at least 14vdc measured across the battery terminals. This indicates the alternator is supplying adequate voltage to recharge the battery.