If you can get a voltmeter and get the van started, measure the voltage at the batteries before starting the van and then measure while its running. When running the measured voltage at the battery should range from 13.8V-14.5V. When the engine isn't running, voltage should be 12.3-12.6V. If you can replicate this, the problem isnt your alternator. If you cannot replicate these voltages, and the voltage remains constant when the engine is running or is not running OR the voltage starts to drop while running, its your alternator.
2000 E350 Cargo PSD w/ UJOR 6" Lift