I do not think it is a short in the Charger. To test it, remove both battery connections and run the maintainer straight across the battery and measure the voltage. If it does come up, then there is indeed something in the Charger pulling it down. The first place I would look there would be the alternator. If one of the diodes is leaking, it could bring the voltage down.
If you have the same voltage results, still being low, then your battery is bad. Not uncommon to find two of them in a row bad.
Craig