Roger,

Some of the early buses did not have the block heater hard wired, it was just a plug that was behind the door where you fill the oil reservoir and could be unplugged from the 120v receptacle. If that's the case, just plug your charger into that receptacle and control it with whatever set up CC used to turn on the block heater, either a button or a breaker in the 120v panel. If you need the block heater, just reverse the process and then change it back.

If the block heater is hard wired, you could put in a switch like Ken suggested.