Considering converting cargo trailer to RV - need advice

With not much room under a utility trailer you would have to have a custom built gray tank. As for a toilet I would go with a cassette toilet, contains it's own holding tank that can be pulled and dumped in a standard toilet. Fresh water tank can be placed inside the trailer under a cabinet, bed, or where ever else you have room.

Good luck with your project and keep us informed on how it goes.