The transit isn't too bad, I just don't think a narrow box is what people who use wide open highways want. Ambulances have transitioned over to the common chassis HD trucks like the 3500 and F350. Even Ram/Dodge, a long time hold out on common chassis designs has a cab/chassis available.
One of my jobs has transitioned to F450s while the other has been using class 6 Freighliners for years.