1987 dodge ram van 150 draining battery

my van was sitting for a month, and did not want to start the battery was dead so i got a new battery it lasted for about a week and started going dead while i was driveing the light dimed and the heater stoped working. was loseing engine power untill i shut off the lights and heater that saved me enuff time to park it (20mins). i can jump it and it start but it does the same thing, just slowly dies. the altinater if pritty new and the fuzes are all good. it is like the battery is not being charged just drained.
does any one have any ideas?

With the engine running and using a digital voltmeter, what is the voltage across the battery terminals.