The fresh water tank sits on top of the floor in front of the trailer, under the couch. Even if it's not there, it should be an easy thing to plumb in some kind of tank.
Black water tank should be relatively easy, depending on the surgery used to install a household toilet. Hopefully, simply installing a new toilet and dump valve will take care of that.
Grey tank placement will be the challenge. A redneck method would be to run a series of PVC pipes back and forth two or three times in the rear bumper compartment, and put a dump valve on one end, and the inlet for the grey water on the other.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.