First question you need to answer is what your are trying to achieve? Do you want to keep it original just install the center bath version? Are you remodeling?
'Moving' a bathroom also allows you the chance to 'change' a bathroom. Requirements around a 1970's bathroom are very different from modern day, and changing from a separate bath/shower, toilet, basin to a wetroom takes up substantially less space. Just keep in mind the old bathroom includes the tanks, so you are effectively moving a large proportion of the infrastructure that goes with the bathroom; not easy when you are considering moving tanks to the same location as the wheel wells. So it is very important that your floor plan includes the fixed elements you need to move, and the other elements (wheel wells, water heater, furnace, fridge etc) that you aren't intending moving. Very quickly a 'move' becomes changing everything - it really isn't ever a blank canvas.
IMHO its why so many trailers are sold 'gutted ready for your vision', that basically means the sellers vision wasn't achievable (sometimes practically, sometimes because it was beyond their skill, or sometimes beyond their budget).