If you ever wondered what is inside a Vanner equalizer, here is the patent circa 1982 - the timeframe when most of our Vanner equalizers probably were produced. http://www.google.com/patents/US4479083
Fig 7 is the schematic for the electron pushers
The concept is simple - a precision voltage divider formed by R12 and R13, a pair of 1% resistors, is connected between the 24 volt input and ground, and derives a reference voltage of 50% of the input voltage. That is used to control the output of an autotransformer that is fed by a switching power supply that produces the output current on the 12 volt tap.
There is no reason a Vanner equalizer can't be used to generate current to charge a 12 volt battery, but you have to connect it properly. One point - it can't be used as an equalizer and a converter (the configuration that charges a battery) at the same time, it can do one or the other.
When connected as an equalizer it has two 12 volt batteries connected, one is the "loaded" battery connected between ground and +12V, and the other is the "source" battery, connected between the +12V and the +24V terminals (loaded and source are my terms, not Vanners). In this situation both batteries get charged from the 24 volt source and the Equalizer forces the voltage to remain exactly equal (within 1%) between them. All 12 volt loads are connected to the loaded battery. If a load is present (you turn on some lights or a radio or whatever) the current to the load comes from the loaded battery, but that pulls it's voltage down below 50% of the 24 volt voltage. The Equalizer supplies half of the load current from the 24 volt source (if the alternator is running) or from the "source" battery if not. That makes it look like the two 12V batteries are in parallel and each is supplying half the current. They aren't really in parallel, of course - they are supplying power not only to the load, but also to the switching power supply that is creating the current to equalize the load on the pair of batteries. The key point here is that there are always a pair of batteries connected to the terminals of the Vanner, they are equal in size and capacity, and they always stay at the same voltage level and are therefore equally charged.
If you connect the Vanner as a converter, you supply it with a 24 volt source and it puts out a 12 volt current on it's 12 volt terminal. In this case, the loaded battery is separate from the 24 volt supply. You do not have a pair of 12 volt batteries connected with the center tap connected to the Vanner's 12 volt terminal, you just have one loaded battery and a separate 24 volt source (which could be an alternator and a set of start batteries in the case of a bus). Here, the Vanner takes in current on it's 24 volt terminal, divides it very accurately in half and sends it out on the 12 volt terminal to supply loads, or in this case charge a 12 volt battery. It will do this up to the current limit of the Vanner. If you look on the Vanner manual http://www.vanner.com/manuals/65-60.pdf
page 8 there is a diagram labeled "Caution Adding 12V Batteries" and the left hand diagram titled "acceptable" is the setup you want. If you look at that diagram, the bus alternator is on the left, the bus start batteries are Battery A and Battery B, and the house batteries are on the right.
I hope this helps. I spent a half hour figuring this out, finding the Vanner patent and schematic, and typed up 75% of this reply only to lose it when I pushed the wrong button on the computer...
Edit: I knew I forgot something. Vanners are always on. They always draw current. If you leave a Vanner connected to a pair of batteries it will discharge them down to zero. It will do it slowly, with perfect equalization, and when you come out to check on your batteries they will both be discharged to around 1 volt. It happened to me... Disconnect the Vanner if you leave the bus without a charger on.