When I bought my 26HE, I had the dealership swap out the Vizio for a JVC TV I had. When I picked up the motorhome, I asked about removing the TV from the wall and the RV tech told me how to do it. "MACNUT" is right about the access panel. You will need to remove the screws and take a peek inside. I was unable to get my hand in there to add the HDMI cables, but I do have a different TV mounted.
It is my understanding there are two bolts that hold the TV mount in place. If you remove those bolts, you should be able to unmount the TV from the wall which will give you access to the back of the TV.
Please understand, I have removed the access panel, but I have not removed the two bolts that hold the TV bracket to the wall. I can only tell you what the RV tech told me. I do plan to install an HDMI cable (or two) in the future.