I went thru every location I can think of and its never been discussed for 5ers. The best time to have one would be when it was going to be built. You can run thru or under 5er but just have to figure how to get to rear of 5er.
Cabinets on back wall and wire chase down interior wall to under the 5er you may get it done.
You can see one installed in the back of this Toyhauler 5th wheel
.