Hi Tom, yours sounds quite different from what I had in mine before I took out all the rotten and mildewed wood.
This picture shows what was nailed down with the original T-head nails, and was prbably original.
The white board at the left of the picture is the front of the starboard cot, and is really very thin plywood over a 1" frame.
Then against the wall is a lip that extends out further than the furring strip -- my guess is that this is the area that had the liftable/removeable lid to gain access to the storage area. The wheel well pretty much ends the storage area.
I think a previous owner of your trailer made the access doors because it's such a pain to lift out the backreast, lift out the cushion, then lift out the lid, to finally get down to what he wanted. I too have thought about access doors but I hate digging stuff out of cupboards at floor level, so I had been thinking of making some slide out or roll out bins. At least that way if I want what is in back it is as easily accessed as what's in front.