Just treat it as one big tank 138 litres I think from memory. The second tank has a jet pump which works on a siphoning effect to transfer fuel. The first half of the tank takes longer to drain as it's using the sub tank. So expect around 550kms from the first half on the gauge . As far as fuel economy how long is a piece of string ! These are rough figures: around town/highway 12.5L / 100
Towing expanda fully loaded 2.3t best of 16.9L / 100 . Worst of 21L / 100 towing into a head wind.
This is cruising at 105 km/h when towing.
Please take into account others fuel economy will differ as my 200 series which has mud tyres, bullbar, rear drawers, full length roof rack, lift kit, Engel fridge and has been chipped, 3.5 inch exhaust fitted which has increased fuel economy slightly from when it was a stock vehicle.
However all this goes out the window when your able to put your foot down to over take a road train while towing the expanda with ease only to look down at the speedo and realized that you have climbed to 130km/h at a very rapid rate.( not that I advocate doing these speeds whilst towing). After 4 yrs of ownership I still can't wipe the smile of my face when driving or towing in the 200 series !
eace: