I do a reasonable amount of cruising in waters with fairly sparse refueling options. Over similar distances to the OPs planned trip too. A few thoughts:
1. Forget the gauges - they're never going to provide useful information,
2. You REALLY need to know the exact volume of the tank/s. BTW, 95L a side seems really small for an 8m RIB, so I reckon they're bigger.
3. Using the engine fuel figures is a good idea - it's good for monitoring the burn if your conditions/plan changes.
4. Having good historical fuel burn figures is very useful for planning.
5. Carrying a known reserve in tanks. I carry a tank/s that give me just over an hour at cruise speed - enough to get into somewhere safe and worry about fuel there. Mollers has a point about fresh non-slip paint - it eats plastic
6. If you can dip a tank - it would be very useful. To make it work you would need access to determine the depth of the tank proper - so you'd need to get the fuel filler line off and dip from there - maybe not viable. Dipping tanks is foolproof if calibrated correctly the first time.
7. Fuel usage can change a lot - bad weather is the main reason but a bit of hooning around in calm weather can dramatically alter your tank level too. I know of lots of people who have run out of fuel. It's one of the main reasons for RNLI callouts to motor boats.