Need advice on accessing engine - 2005 Horizon 40AD

Hello folks, I searched for this topic and did not see it anywhere but do apologize it has been discussed before. I have some wiring to do that requires I find a source of power on only when the key is in the on position. This power will only be used as a signal source to a new relay I am installing in the electrical bay of my 2005 Itasca Horizon 40AD. As the electrical bay is right beside the engine on this rig, I figured I could find a close easy source there with the engine. Problem is, the only access hatch I have ever opened before is the small one in the closet and that does not expose enough of the engine compartment to do what I need to do. Does anyone know how to get to and open the larger access panel under the raised carpeted area between the closet and the bed? From the bottom side, I can see it is held down with screws the same as the smaller panel that I already have open, but I can't figure out how to remove the carpeted step covering it. Thanks in advance! Gene in Arizona.