Perhaps some one can answer that owns that model currently. I owned one until 10 years ago, 96 Dodge 19P.
The "plug" is a little hollow thin plastic gadget shaped sort of like a thimble which is inserted into the outlet. There are no threads, it just holds by friction. It has a small hole about the size that would be made by a 1/16 inch drill bit in it that allows air to escape as water is added from the shore line hose. Of course, it is simply pulled out to fill from the door with a hose and a filler tube.
I remember a lengthy thread on this B forum a couple of years ago about the source for these plugs. An archive search might find it. My suggestion is simple: go to Home Depot or Lowes to the hardware department and look for a rubber plug of the appropriate size. It need me only an inch or so long and should slope, being bigger at one end than the other so it can be inserted. In other words, a cork shaped rubber plug. That failing, just use a plain old cork but drill a little hole in it lengthwise to allow the air to escape. I would suggest a 1/8th inch or so drill since cork tends to close back up. Any plug you make will need this hole for ventilation to allow air to escape and enter.
As to the back panels, I don't recall but I suspect they are just like my current RT. I removed the top back panel to add wiring to a backup camera because I wanted it to run full time, not just when I was backing up. I removed the panel and got the power from the light over the back (twin) bed. On mine, there are little round "buttons" of upholstery. You can take something like a screwdriver or case knife and pop of their head CAREFULLY. Underneath, you will find a screw head which can then be removed to remove the panel. Just take time and care.
PSW
2013 Phoenix Cruiser 2350
2014 Jeep Cherokee behind it
and a 2007 Roadtrek 210P for touring