You can use a utility vehicle to cover a heavy rescue slot on a call. Both vehicles (the other being a heavy rescue) do the exact same thing currently.
I have about 25 Utility Vehicles and 2 Heavy Rescue (up until the Rescue Engine come out) as 80% of my fleet of pump engines had rescue capability (thus were Rescue Engines). There was only 2 heavy rescue units based at HQ. So I could cover HR requirements I use Utility Vehicles and have done as far back as I can remember. Now it's not too bad with Rescue Engine but I still use Utilities.
The other vehicle that covers a role you may not realise is the Mobile Command Unit also covers a Battalion Chief role too, so if there is a requirement of one of each on a mission, you don't need to send a Battalion Chief separately.