I bought a 14 ft cargo trailer spec'd like I wanted to convert to a mini toyhauler. Didn't want a full size toyhauler, but wanted something big enough to haul 3 bikes, , shower, etc. and small enough to use for day trips etc. and room to sleep for two. Mine has sleeping for two, dinette, hauls 3 bikes, fridge, stove, microwave, water heater, 20 gallon fresh tank, 40 gallon grey tank, shower, AC, toilet forced air heat, plenty of storage for food etc. outside awning. A single 5 gallon propane tank, two GC2 batteries on the tongue.
Was fun to build, has been in use since 2004. When I built mine it really wasn't any cheaper than buying a small trailer, shell trailer wasn't real expensive, but stuff like furnace, AC, stove, toilet etc. etc. adds up pretty quick even if you find used stuff. I knew that would be the case from the start, but it was a solution to what I needed.
As mentioned, cargo trailers are NOT lightweight. My 14ft is rated at 7,000 GVW, and is typically around 6,000 with two bikes in it. My 30ft TT with dual slides is 8500 loaded up. Unless you go real spendy, cargo trailer frames and wall framing is all steel, roof framing is steel, roof if galvanized sheet metal, outside walls thin aluminum. Makes them pretty heavy to start with. And the walls are usually only 1/2" or 3/4" thick so minimal insulation.
For years in addition to use as a toyhauler for motorcycles, we used it as our travel trailer as well.
Overall, if you want a fun project for a custom trailer and have the tools, skills and time, go for it. If you want it because it's the only solution to what you want for layout, go fo it. if you think it's an inexpensive way to get a trailer........ Your barking up the wrong tree. If you think it is a way to have a lightweight trailer.......... your barking up the wrong tree.
2011 Keystone Outback 295RE
2004 14' bikehauler with full living quarters
2015.5 Denali 4x4 CC/SB Duramax/Allison
2004.5 Silverado 4x4 CC/SB Duramax/Allison passed on to our Son!