I took a closer look this afternoon. The undesired opening into the interior space from the rear service bay is built into the design/manufacturing by Winnebago. The bed has not moved; it is rock solid. The opening appears to have been closed off with a piece of foam material. On the inside there is a tape of some sort to hold the foam in place. I can not tell if the tape was part of the original install. The foam appears to be part of the original install as the same opening is also closed off with foam on the passenger side. The passenger side foam is still in place.
To my way of thinking the foam is very light material for this job. It is open celled foam that you might see in packing crates; see the photo below. Wonder why a more substantial material was not used. Maybe some type of rubber.
Unless someone has a better idea, I am going to try to find some rubber that will fit and then glue silicone it in place from the outside thru the rear service bay. I do not think removing the head board will be necessary as the opening is below the headboard and adjacent to the bed frame. I'll just slide the mattress out of the way. I may be able to silicone the inside by wiping the silicone on by hand to help bond the two joints. Since the space between the bed frame and the night stand is small that plan may not work. If that does not work, I may have to use a tape. I'll try to find something more substantial then the piece that I found there.
As an act of preventative maintenance on the passenger side I will silicone the outside. The inside is hidden by the AC return and will be a bear to reach.
Below are photos showing the driver side problem.
You can see the snake I shoved thru the opening from the inside at about the floor level.
This photo is looking at the opening which is the entire space you can see in the photo below.
In this photo below you can see the piece of foam that fill out of the opening and laying on the floor.