Ambulance difference to regular sprinter… probably has an inverter already inside, proper insulation in the walls.
In a more serious note, insides of any ambulance are usually filled with storage lockers etc, that there is not that much room inside. If you feel you can build yourself a decent bed/table/whatever you want without stripping everything off… why not. If you plan stripping it anyways there are probably less abused options available than old ambulance.