Since you can't pull the heater without disconnecting the water lines (and the wiring too), I don't see how the outside approach can really help. Unless there is a lot of slack in the lines, and that was never the case in any of the several RVs we have owned.
Cabinetry is rarely complicated in RVs, but you will just have to crawl under them to see where the screws are located. PITA, but probably not complex.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL