If you use (2) 12 volt batteries wired in (parallel) you will have 12 volts with the amperage of each battery available for a short time. Unless the batteries are deep cycle batteries they will not run the coach on purely battery for as long as 2 large 6 volt deep cycle batteries wired in (series) will. Large coaches have (4) 6 volt batteries wired to provide 12 volts to the system. They are high capacity deep cycle, deep discharge batteries. Mine will run for days providing inverter power for TV, vents, lighting and such. Never saw a motorhome wired for 24 volts but I never saw a million dollars either. Careful when you decide on how you will proceed with your batteries. It only takes a second to undo many $$$$$$ of things in you motorhome.
2003 Newmar Kountry Star 3905, Freightliner XC chassis with CAT 330. Winnie the black lab, pretty Airbus captain wife, retired airline pilot with 11 grandkids. UH-1 pilot (Huey) U.S. Army 1967-1983. RVN 68-69. Northern Idaho my summer home.