welcome to the forum its a great place to get your questions answered. I can't answer you question on the specific rigs, however in my case the availability of portable totes for potable and waste water seem a much better solution then attempting a tank change out.
tank size on a rig is usually a compromise between structural, cargo capacity, and gross weight change increase on you have to decrease something else to stay within limits. JMHO
Jake my sidekick (yellow Lab)
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho