If the underside of the home is covered with a one piece plastic sheet (insulation underneath) then installing a bath would require cutting this. In my experience it's almost impossible to repair this sheet properly. That means if you are in a cold climate air might leak in and freeze the pipes in winter.
But to answer your question, anything can be done if someone is willing to pay the price. I would look around for a plumber who has worked on mobile homes and get an opinion and estimate from him. A few residents who have been in mobile homes for years should have names of mobile home experienced plumbers.