I have thought about this as well, just not quite there yet as I am still working to finish my build. You may already be aware, but for a hefty price and with poor reviews, the dometic extend a room seem to fit the bill if you have a ramp:
https://www.amazon.com/Dometic-954091-0 ... B0031NHRXQBut at that price, I would almost be inclined to draft up my own design and run it by an upuolstry shop to see if it's doable. You can probably source the canvas for pretty cheap and fashion up the suports yourself.
I have a friend that had his pop up material replaced, which he ordered through bear crek canvas:
http://bearcreekcanvas.com/index.cfmThey did a great job, used better materials than used at the factory and even had a tinted window option (which he went with and looked great). They or somebody like them may be able to do this type of project. Ultimately if you wanted to do it up nice and clean, you would utilize the exact same anchor strip and channel that is used in tent trailers.
There are options out there, just gotta shake the bushes a bit and see what pops up.
-Domo
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