You may want to stick your head in there with a flashlight, as the outlet may be hard to see. On my 36RD the outlet is at the top of the compartment, facing forward, on the front face of the black metal wiring compartment with the solenoids, etc. It's possible that the block heater was never plugged in, in which case the cord may be tucked up out of the way.
You can tell whether the block heater is working by watching your amp display (as long as you're not plugged into a 50amp circuit) on the OnePlace panel when you turn in on. If you are plugged into 50amp you won't see an amp readout, so then you'll need to unplug and use the invertor or the generator and watch the amp display when you switch on the block heater.
2005 Winnebago Vectra 36RD