RV Campers are built to last for years, not decades. So, they are built cheaper than a rugged box trailer that is built to haul cargo. And it is much cheaper a new box trailer and built it up yourself, then to buy an used RV.
I will start with "Ground Up" tutorials. Meaning, I will start with the tire size and ratings that you will have to comply with to meet the GVW, GAW and minimal tires to consider and number or axles you need to transport your trailer and towing vehicle on all roads you will travel.
*GVW = Gross Vehicle Weight *GAW = Gross Axle Weight