Usually they use diesel heaters on the ev busses. Sucking power from the battery bank to heat the bus is definitely not a good idea. I'm part of a team that is working on ev school busses and other ev projects and are on the edge of a new way to generate heat without using diesel or sucking the battery dry. It will change a lot in the industry when proven. Ev's have their place but are not the be all end all. Some applications they are way better and others they suck