Im bias but I think the stock trailer is the best option. The first time we camped in a stock trailer it wasnt much more than a tarp and heater and it was not uncomfortable. So starting from a very basic somewhat livable space, all ive done is make improvements. My neighbor set up his cargo trailer into a similar set up very successfully.Im either going your route or going to find a old boat trailer and build one from scratch.
Either way youll have to post some pics once you get yours going!