1996 Pontiac Transport Power Drain When Cranking

I was not the primary driver on this vehicle. The vehicle sat for about 3 weeks and needed a jump. It sounded very rough when I jumped it and I was told it had been running like that for a while. The negative clamp(grounded properly)started smoking for a sec when it was cranked then stopped as the engine caught. The vehicle drove about 1/4 mi. And died (ironically by a used car lot, who were kind enough to let me try their jump box when a car jump didn't work) When I tried to rejump it I had a hard time getting a good connection. When I did get a connection, all the warning lights came on and the voltage showed around 12 on the dash gauge. When I cranked it, it turned half way and the voltage dropped to nothing. I checked and replaced the battery which was weak. It still does the same 1/2 turn and drains the voltage, so I unhooked the new battery for now. This van was/is 60 miles from home and I had no diagnostic tools. I will go back tomorrow and try to get it running enough that I can get it home and work on it properly. Do you have any suggestions on where to start?