modeling /cad drawings of space...
To truly model the trailer you need to include the frame in your model as it impacts where you can locate things that need to access that space.
Things like wires, plumbing, gas lines, air vents, etc. have to be carefully planned in order to not have to drill into frame parts and provide access to the correct spaces. Between the frame rails is open the length of the trailer, outside of the rails has limited access to certain areas... ie its own "box".
Drilling holes in the frame can weaken it... so avoiding those boxed in areas for access is the best choice. Some cannot be avoided but good planning makes for minimal alterations. I just finished this aspect of my trailer renovation so I am keenly aware of what I was less observant of.