If you never boondock, and you don't have a residential fridge, then multiple batteries are a waste of money. The only time the battery will be "needed" is to provide the extra amperage needed to operate the slides and landing gear/levelers and that is for a short time. The battery and truck charging system should maintain the 12vdc needs to keep the fridge and any other low voltage needs going down the highway.
2012 Laredo 303 TG
2010 F250 LT Super Cab, long bed, 4X4, 6.4 Turbo Diesel