The ambo shore power system usually feeds to a box where it splits off to the inverter, inside outlets, battery maintainer/ power source, and the block heater. Now if you wanted/had to you could run the block heater off the inverter (batteries), but normally it is wired parallel and simply comes on when the rig is plugged in. Optional equipment can include a disable switch so you don't waste power or pop a breaker when its warm out. Personally a plug system is a better choice than a switch because you can elect to heat the block off your house batteries if needed.
When the unit leaves the manufacturer, it's intended use is as an ambulance and it uses normal automotive batteries, and not deep cycle or 'house' batteries. The OEM installed inverter, i.e. the Vanner brand, is normally not used unless the engine is running, thereby preserving the battery power. If the engine is already running, then there is no need to operate the engine bock heater, right?
I don't know the exact specs, but a block heater is a heat producing device and therefore has a high amperage draw (1,000 Watts average). Operating it through the inverter for the half hour or so that it would take, could possibly draw down the starting batteries enough to make the whole process null and void. If your starting batteries are fully charged and your glow plug and relay system is operating properly, you shouldn't really even need the engine block heater except in extreme cold.
My point is, the block heater can be wired anyway you want it to be, but normally you want it to draw power from somewhere such as shore power or a generator, and not from the on-board 12VDC system via the inverter, unless you do have a rather large 'House Bank' which is isolated from the starting system. Although, this could provide a last-ditch emergency procedure if all else fails.
As Alex said, normally it draws power from the shore power system. Ambulances are designed to be plugged in to 120VAC power anytime they are parked at their station waiting on a call. This is for charging the vehicle batteries, charging the portable electronic medical equipment and portable radios, and in some cases the engine block heater.
Have you ever noticed that anytime you see an ambulance in the field, at a vehicle accident, residence, or business, it's always running even if it's emergency lights are not operating? A lot of departments have a policy that states you will not shut down the engine unless you are in a long standby situation, such as at a sports event, etc. and conditions are favorable for restart.
Here is something you guys might want to take a look at. It addresses the same topic here:
http://www.ford-trucks.com/forums/295560-12v-block-heater-2.html
And just FYI, it heats the coolant, not the oil.
http://www.thedieselstop.com/forums/f30/block-heater-replacement-183048/