hi i'm billwelcome to just answerplease remember I am NOT there in front of you! my answers are based on your information.
that is very possible BUT not a common problem with your year van, more likely teh system is either under or overcharged, you will need a set of gauges to verify the proper charge and pressures, DO NOT just add refrigerant without a gauge set it is VERY EASY to overcharge the air conditioning system, as for where the refrigerant went IF it is undercharged? evaporators were very common in that generation of minivan 134A was still new ti everyone and we still allowed the evaporator to sit vertically (we put it at an angle now) and it was prone to corrosion.
The system hasn't been touched since it was new , so it shouldn't be overcharged.
ok then an undercharge would be very likely BUT to verify this you will need a gauge set, if you do not have the proper tools to service the system then i would strongly recommend a pro, the cost shouldn't be more than 100.00 to do a complete evacuation draw the system down to a vacuum, measure the amount of refrigerant that was in the system, recharge the system and add a leak detection dye and top off the compressor oil, and test for leaks and proper operation. IF there is a leak the pro has the proper UV light to detect exactly where it is and give you an estimate on the repair.
the air conditioning system is NOT something an amature should work on since teh refrigerant is very dangerous if accidentally released.