You can use a similar design to your existing fleet without a literal retrofit of an existing chassis to shoehorn a battery and electric drive train in there.

The retrofits usually are less preferable not only because of pointless inconveniences like transmission tunnels, but because they'll be the manufacturer's first toe dipped into the EV waters. The retrofit chassis speaks to either a rush to market, or a cautious approach not wanting to commit too many resources. The former says it'll have issues, the latter says they might bail on it and leave you stranded for service and repairs. Or both at once.