2011 Pajero Sport 3.2 Fuel gauge inaccurate

[Honey Badger]

We have the regular 70L tank and an aux tank above spare wheel (about 30L). Needle stays at FULL mark until aux tank is empty. I found that the range I got for aux tank was a good 120+km more than expected. Fuel gauge needle goes up and down with ignition on and off as per normal, just give incorrect readings. If I shake car then I can hear some fuel (quite low) but needle says 50% tank.

Everything else is in vehicle is working fine. Is the first place to look inside the fuel tank at the level / float sensor or should I be checking for something electrical? Many thanks for any tips...