There is nothing worse than 3D CAD Model inaccuracies when heading into manufacturing. For over 3 years I was contracted as a Mechanical Engineer for a company whose goal was to take standard 20ft and 40ft ISO Shipping Containers and turn them into mobile data centers. We downloaded a free 20ft ISO Shipping Container on one of the Free CAD model websites and started using that for our design work. It was accurate enough. It had the right dimensional footprint, the corner blocks looked correct, and it had corrugated walls and a corrugated roof. What could go wrong? It wasn't until we got into the manufacturing stage, when the models inaccuracies reared it's ugly head and become a daily problem solving, conference call with the fabricator. The corrugation pitch was off, the interior dimensions were short, and the underneath C-channels we used to anchor components to and route cables through, were in the wrong locations.
My fellow ME's and I wasted countless hours that could have been saved if we had a dimensionally accurate model. We had conversations about how much we would pay for one, if we could find one.
I decided to spend my own time in the container yard for several weeks. Crawling on top, inside, outside and underneath the containers. Measuring and documenting every feature to produce a dimensionally accurate 3D CAD model for each size ISO Shipping container. When the company decided to move forward with the design work on a 53ft Shipping Container design, we were way ahead of the game.
I spent the same time on the Enclosed Cargo Trailers I used for Solared Survivor. I'm positive that you will find a free 3D CAD model or simply make one yourself, that has the correct overall dimensions that you can use for your design layout. The questionable part is when it comes to fabricating one and you need to be accurate when mounting components to the trailers frame members which are hidden behind the wall and floor panels.
It comes down to what your time is worth. The time you spend fighting the inaccuracies of a free model with the fabricator or the time you spend accurately measuring and modeling it yourself, vs starting with a dimensionally accurate canvas for your design work.