May get better coverage in National RV owners forum. You are correct the three batteries under the step, two for coach, one for chassis. The fuses under the dash are for the chassis only. You will have a 12V distribution board for all the coach power. Mine is in the basement compartment between the entry door and front wheel. Your not going to find any wiring schematics that show you how to connect the batteries. Best way is to trace the cables to the starter, 12V distribution board, frame, and help from the owners forum. Which chassis do you have?
Chuck & Teri, 1999 35 ft. Dolphin 5350, F53 Chassis with tag axle, Chassis build date 1/99 in Mexico, Banks System, Trans Command, Air Lift Air Bags on rear, Koni Shocks, Blue OX TruCenter, TigerTrak track bars front and rear, Roadmaster 1-3/4" rear auxiliary sway bar, 2013 VW Passat TDI SEL Premium, 1994 36 ft. Avion 5th Wheel. FMCA #F430129