Yes, that is the water drain. The line attached should either disappear below and dump to atmosphere, or lead to another valve (not sure about the 93 van). The "normal" FM80's will have a plastic valve, like the top bleeder, on the side, at the same corner as the drain nipple. The 93 6.5's (FM100) have a drain hose leading to the front of the engine, with a needle valve located there. Yours may be a hybrid, of sorts. Follow the drain line, to be sure. Something stops the flow, either at the fuel manager, or along the line somewhere. Accessibility in the van engine compartment may have a play in it. GM considers water drain opening a regular maintenance step during normal periodic service, so accessibility would have been considered.