SAS--Self adjusting steering?
Sorry I could not resist the jest.
Yes there should be a switch on the dash that is for the gas tanks, unless they took the cheap route and put a lever on the floor beside the drivers seat next to the door. You could take it to a gas station and fill only one tank. If the gauge shows full you know that is the tank being registered, if not then you know it is the other tank. If both tanks are full drive for a while to see if the gauge goes down, if it doesn't and non of the switches make a change it is time to chase wires from their respective tanks to the dash, or take it to the dealer and let them figure it out. Many RV places do not do chassis work so you may end up with a mechanic shop that does work on RVs. I take it you do not have the owner's manual?
Berniece & Russell with LilBit the Netherland Dwarf rabbit who knows she is queen
2003 Kia Sedona