I have had 2 trailers that used the frame from Popup Campers. The first I built into a fully enclosed trailer. One change that I made was put a longer axle under it moving the wheel outside of the frame giving more flat bed space.
The sad part with this trailer is that it was lost because of a drunk driver that did not want to stop for a stop sign. If it had not been for this trailer we probably would have been kill, because this accident was also a hit & run which took the whole right side of my wife's truck. One thing I learned from if you have a trailer of any value it needs to be insured with your home owner insurance on a special writer. The second trailer is a flat bed trailer that I bought on EBay for $85. Since I still had the parts from the old trailer so the first thing was to swap axles moving the wheel to the outside of the flat bed.
This picture is how it was when purchased & I have not taken any pictures after I have put it together. With this trailer I have hauled garden tractors, tools, & other items when making trips to my moms place. This trip is a little over 200 miles one way. The only problems I have had was with tires which I learned was caused by the way I had mounted the fenders. I corrected this problem by mounting them differently, so far no problems (knock on wood). I have found the frames on these trailers is as strong if not stronger then most utility trailers found on the market & usually cost a lot less.