I have an '05 Bounder 36Z, to remove the doghouse access plate there are three phillips screws, remove them and the plate comes off. Just make sure the rubber gasket is in place when you replace the plate.
The ECM unit on mine is right in front of the engine accessible from the front. Drop the "hood" and it's almost in your face. Not sure why yours is so difficult to get at.
I have factory installed speakers in the bedroom slide out and the wiring for them is accessible from under the dash by the radio. Don't know about your rig.
Good luck.