Doubtful on commercial built custom 20ft RV trailers, the cost to build "one offs" is astronomical, leaving one to have to charge multiple times more than a commercial built cookie cutter trailer that flows down an assembly line.
You are going to have to contact some specialty builders like concession or horse trailer builders and pitch your idea.. Horse trailer builders often will build a a living quarters section on the horse trailer so they should be able to make something like you wanting.
Concession stand builders typically custom build the trailers to fit what the customers needs are.. The may be willing to adapt a bit to living quarters instead of commercial cooking..
But, honestly, you are camping, and you specified 20ft length, save money, time and lost hair and go find a normal travel trailer with a layout you can live with.. There is very little you can do with 20ft in terms of layout as there will be concessions you will have to make to fit it into that footprint..
Basically you will have only enough space for a small bathroom, front or back side door (unless you want a rear opening door), kitchenette, small dinette/twin bed and small couch/full size bed..
I had a 20ft TT, there is not much you can do with spicing that space up to a luxury hotel room..
![](https://i.imgur.com/p5N9XBzl.jpg)
![](https://i.imgur.com/D0EYpTvl.jpg)
![](https://i.imgur.com/bZ0X3FNl.jpg)
![](https://i.imgur.com/Lr3xcZ4l.jpg)
Those pix are from my 20ft TT after I rebuilt it due to severe water damage and rot (I bought it water damaged)..
There is nothing magical with 20ft of space to make it worthwhile for a highly customized one off RV.. I lost a lot of money and personal time on that one although when I rebuilt it, I did a lot of customizing to us and didn't intend to sell it.. People liked it, but were not willing to part with a few thousand more.. Took a long time to sell and lost thousands..