I spent 10 in the U.S. Air Force working in Fuels, then got a civilian job on a pipeline.
In the AF, we used master meters and hoped for the best with our meters. Our tanks had been calibrated mathematically, I assume by measuring the circumference of the tank every so many increments up the side then figuring the area and volume. This is difficult and has a margin of error. We also used API tables to compensate for thermal expansion.
You might have better luck using water and a turbine meter with a prover loop. You can look up prover loops online, they usually are set up with turbine meters. I looked in to this online awhile ago and saw a trailer mounted setup, so you should be able to find a company that will come with a complete unit on a trailer.
The way the prover loop works is by comparing how much fluid passes a known volume of pipe to how much went through the meter. They use a calibrated section of pipe in a loop and either a plug or a ball that slides (not rolls) down the pipe. The ball does not allow the fluid to pass by and sets off sensors at the start and end of the run. The known quantity between the sensors is compared to the readout from the meter.
Unless you always filled your tank to a certain mark you would still need strapping charts for your tank. If you can find a company that has a portable prover unit, perhaps they would draft strapping charts for you. If not, you will need to accurately mark the tank at increments as it is filled. I would look into a automatic tank gauging system that you could move from tank to tank because you should only have to do this once for each tank. They make some nifty laser tank gauging devices.
Here is a link to a pdf I found with info on prover systems and it has an image of a truck mounted one. Good luck!
