These trailers are not quite the "blank canvas" that you might think they are. They fit together like puzzle pieces, and just because you have removed the pieces doesn't mean its easy to start over. For example, You will have various holes in the shell, like for the plumbing vent stacks, the stove vent, the refrigerator chimney, the furnace, and the water heater. If you decide to move all these things around, then you have to patch up the old holes, and create some new ones. Closets and cabinetry are strategically located to hide vent pipes, and ugly appliances (water heaters, converters, furnace, etc.).
The weight issue is a major one as well, and not only for the quality of the towing experience. The 70's era trailers are already known for rot and rear end separation. Positioning a lot of heavy appliances back there is only going to aggravate the situation.