I've got a very slow battery drain somewhere.
Can anyone think of a reason the battery won't charge while hooked up in the car but will when it's sitting on a block of wood on the floor?

 

No; unless your drain is more current than your charger can put out. In which case it might not be able to charge the batt at all but instead might just barely keep up with the drain.

Check things like lights for the drain... underhood light, interior lights, tonneau light (right at the hatch latch)

 

I think that's exactly what's happening. I'm going to hook up the volt meter and start pulling fuses today to see if I can find it.