I built a a cargo trailer into an RV, and it takes a lot of work to do it right.
As others have noted, off road worthiness is the issue with a cargo trailer because of the low ground clearance, so I purchased as Smittybilt trailer and selling the one I built, as we want to drag the trailer on every trail, not setup a base camp to which we will return at the end of each day. Decide what kind of terrain you are going to drag your trailer through before you lay down any cash...