Most joiner work will need to be custom designed and made.
Not what boatyards usually do. R&R parts is all most attempt to do.
I would look for a local RV or trailer or bus conversion shop and have a talk with them.
Many will job out furniture mfg and thats the folks you need to find.
If this is your first design attempt it will pay big dividends to mock up the new interior in 1/4 ply or cardboard .Most folks dont think in 3D , and actual use may change many dimensions.