I'm in the process of building one. Started with a brand new 8.5x20' enclosed trailer with 7.5' ceiling. Stripped it to the frame, spray foamed the roof and the under carriage. Used ridged foam board insulation for the sides because I was concerned that the heat from the spray foam would warp the sides. I sealed the roof with a silicone sealant and installed 1500 watts solar on top. I have a full Victron inverter setup so it's seamless going from shore power to solar. I have 2 bunks in the v nose up front and plan to put a queen bed on a platform in the back. (platform will be able to be lowered from the roof down to sleep. I'll be able to store it against the roof when not in use so that I have room to park an atv dirtbikes kayaks etc underneath for transportation) I have a bathroom with a composting toilet and shower, kitchen with propane stove and a propane water heater. I have a 42 gallon freshwater tank and I have room to add another one if needed down the road. No onboard black water tank because I felt a portable one was a much simpler solution. I put in 2 windows and have plans to put in 2 more small ones. I have a mini split that needs to be installed to provide heating and cooling. I put a ton of work into it this spring and summer but my fall has been so full. Still need to do a lot of finishing touches and the little details that take up all the time. The plan is to be able to completely off grid for weeks at a time.
So far I'm in it for around 12k and I probably won't be spending too much more since I have most of the supplies I need to finish it. I bought lots of stuff used or new stuff for dirt cheap on eBay. Also got a hell of a deal on the trailer so that helped. The reason for going this route was that the price of travel trailers are absolutely ridiculous right now. Going this route let me choose my layout, absolutely make sure that everything was sealed off properly and done right. I shouldn't ever have to reseal the roof or worry about the trailer rotting away.