One of my friends works in a shop that does mostly mostly light/medium duty trucks and vans, municipal, police, fire, ems, and refuse. He's said over the past few years there has been a big swing away from the diesels back to gas engines. Cheaper to work on, usually pretty reliable for the length of time they are kept. Said the 8.1L chevy engine and the Ford 7.3L has basically become a mainstay for the truck and van based chassis they work on. Said one of the municipal refuse companies has nearly completely switched to natural gas engine to get away from the emission/ after treatment problems they have with the diesels. It's not so far to believe school busses wouldn't follow suite, makes perfect sense these days.