2008 Chevrolet Uplander Power

The van would not start, I got a boost, it started, then the 13.0 Volts started to decrease as I headed toward Wal mart. I kept all power off as I drove half an hour to get to Wal Mart and just 5 kms before arrival, all dash lights went black and blinkers did not work & the idle was revving up and down, and so I glided into the parking lot. Bought a new battery, installed it, repeated the problem on the way home. Tested the alternator (removed the positive post while van was running & it quit) & thot it was needing to be rebuilt. Took it out, had it tested, all was good. Also replaced the starter fuse, removed and replaced the starter (was told it was fine and did not need to be rebuilt). So now we are baffled as to what is the problem. Have even tried a direct line connection from the alternator to the battery to test if its the wiring, but that did not work either, the power kept decreasing as the van stayed running. Any ideas, please?
This is the first van I bought brand new, had an accident less than a year later, took it to the wrong body repairman because he did not write it off and I have had nothing but problems ever since; the rear suspension has not worked and I have complained that its not working to this guy and he has not fixed it properly ever since. Any suggestions, please feel free to pass them on to me. Family members are helping my husband work on this thing to save us money as it has put us in the hole completely and I still owe ONE more year of payments! Help.