Sounds like you're losing (or lost) the ground bonding from the engine to the body.

 

OK, I just checked all the grounding wires from the battery, making sure all was well connected. It all seemed good. I then started the van. Sounded good. Did key on/off and all three codes are coming up again!Aaargh!!

One other thing I've now noticed: The negative wire to battery is as tight as I can get it, but I can easily pull it completely off of the negative terminal. Could that be the problem with the 'switched battery'?

 

That could be all your issues. Eliminate it as a possibility.

Replace the battery cable, or jam a screw or two between connector and stud.

Weak intermittent grounds are the source of 85% of electrical issues.

 

There are other grounds for the vehicle. Check the ground straps from the engine and tranny to the body vehicle frame.