You will have to use a “boost” converter to get the voltage up to 72vdc, plus a few volts more to push some current.
http://en.wikipedia.org/wiki/Boost_converter
You have to know the charging characteristics of your target battery so you know how to taper off the charging.
You will need a monitor circuit for your source battery to stop charging when the source battery is to low, or you will ruin it.
Your PI circuit will need to be powered by a low voltage source that you will have to provide from the battery, either an “buck” converter or an inefficient linear converter.
http://en.wikipedia.org/wiki/Buck_converter
5 volts would be the best starting point. Hopefully you can find some logic level fet’s for the pwm so you only need one low voltage supply.
You will have to decide if you want reverse polarity detection. If the input battery is reversed the charger will not run. If the charging battery is reversed your circuit may blow up or the battery will be ruined.
All of the converters you need can be simple non isolated one transistor circuits. This would be a plus for you because you can have a common ground through the whole system. Realistic power levels for a boost converter are around 150 watts. So you could get 2 amps charging current. If you want much more then that then you will have to use the push pull you mentioned or a forward converter, however this would probably quad-drupal the complexity.