When I first got my VMSpc software (and later the BlueFire app), i kept track of the miles driven according to the odometer, and compared it to the miles driven according to my GPS. The miles were very close. I also compared the fuel I added at the pump to what the software gave for fuel used, and again, it was very very close. Sometimes a little higher, sometimes a little lower, depending on how level I was when refueling. Computers are pretty good at calculations, so I will go with the software calculation. I reset my trip meter with every fuel stop, and take a mental note of the result.
I don't have an Actia gauge, so I can't speak to accuracy of those.
BlueFire also lets you download the trips as a CSV file, so I can review the MPG's for a trend over time. Good enough for what I want to see.
'04 Alpine Coach 36' MDDS