Measure fuel with UZI , CANBus , Volvo fh12 truck

Hello

First I'm from Bulgaria, and my name is Radoslav Raykov. I saw that Wialon offers everything what I want and to be most exactly i want a few things

1) to track trucks in real time of course and to can trace their history
2) to be able to using garmin navigation for dispatching and two-way comunication with driver
3) And of course the most important - to track the fuel consumption of the truck

I want litle bit help from people who already work with these functions successfully to give me advice on what gps tracking devices to use. To achieve these requirements.

The trucks are Volvo fh12  (from 2004 to 2010 )one truck has 2 tanks, one 800 liters and one 400 liters. Are there people who have successfully installed the GPS tracking devices of that trucks and successfully measured him fuel consumption ?

Generally I do not want to drilling the fuel tank and to install additional sensors (to not break the Vehicle warranty)
But I would be happy if people who have done this on this type of truck to tell me how many additional sensors require one or two?
Or if someone uses the original sensor from the truck dashboard, can tell me method to calibrate the fuel tank?

Most will be happy if someone said that was able to measure fuel consumption through CANBus
Another option is ultrasonic sensors like " UZI" - How many sensors are needed and with what gps tracking devices are compatible ? I mean can I monitor the fuel consumption of truck like Volvo with 2 fuel tanks on the truck and one fuel tank of the aggregate (of the truck trailer)

I would be very grateful for your help. Thanks in advance.