Good luck !
Likely you will have to employee at least 2 companies. One to build the frame/chassis and perhaps the body and another to build the interior. There are a lot of companies who build utility trailers, but they don't really know how to build something that does not look like a box !
Most of the interior can be done by a custom cabinet shop, but they may not want to touch plumbing or electrical.
Personally, I have been mulling this over in my head for several years. Here are my thoughts.
• No black tank. Use a cassette or composting toilet.
• Unless you plan to do a lot of boondocking, skip propane.
• Solar is always a good option.
• Spend the money upfront and install a inverter/charger/transfer switch.
• Also, bite the bullet and buy lithium batteries.