* Check the sticker: Look for the sticker on the driver's side doorjamb or the glove compartment. This sticker will list the GVWR (Gross Vehicle Weight Rating), GAWR (Gross Axle Weight Rating), and the towing capacity specific to your van's configuration.
* Consult your owner's manual: Your owner's manual will also specify the towing capacity based on your van's options.
Without knowing the specific configuration of your 1999 Dodge Ram Van 3500 (engine, transmission, etc.), I cannot provide a reliable towing capacity figure. The capacity could range from several thousand pounds to over 10,000 pounds depending on the options.