If the tank is under the bench seat, open it up and try to see where the lines lead.
Mine is inside the stub wall behind the driver's seat with a little wooden door. If you did not know it was located there you might never find it.
Best of luck finding yours!
Full Time in a Kustom Koach Class C 28'5", 256 watts Unisolar, 875 amp hours in two battery banks 12 volt batteries, Magnum 3000 watt PSW inverter.