They don’t need to be accurate to be useful, just consistent. It doesn’t matter if the readout is off by 50% or more, as long as it’s the same all the time. Then any variation tells you there may be an issue.
The gas stations in your area might have recently switched to winter formulated gasoline. That, combined with cooler weather, might account for the decrease in fuel economy.
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Have you only owned it for three weeks, or that’s when you started noticing a change? I’ve owned a dozen full size chevy vans; 2 diesel, 1 305, and 9 350 (5.7L)'s. The diesels of course got better than 18 MPG, but the 350’s with combined city/highway driving and a pretty decent load got 14 MPG minimum. Now, if you’re talking about a box/delivery van with duals, then yes, 10 MPG is a gift.
I have a very strong suspicion this is not a box van, because those would have a 6.0 V8 and 4L80E transmission, not the 4.8 V8 and 4L60E
And it’s almost certainly a 1500, as that’s the only one with the small V8 and small transmission
And anything with dual rear wheels would have the bigger engine
I bought it new. cargo van express 2500.Even winter time it was never below 10.mpg.
After spark plugs and cables done i should be getting 11.5mpg , but i’m still stuck at 9mpg.
I’ll check the caliper, also the car has no check engine light on , so will the OBD 2 code reader be of any help?
We service a small fleet that has several 2500 pickups and vans with the 4.8. The 4.8 sure isn’t very popular to the public but I think that fleet service vehicles have them for some perceived gain in fuel economy.
But on the whole that family of engines is one of my favorites. That old pushrod technology still works.
If either the thermostat or coolant temp gauge isn’t working , and the coolant is a little colder than it should be or the computer thinks it is, higher rpm and lower mpg would likely result. It doesn’t take much of a temperature difference, might not notice it on the dash gauge.
All of our 2500 Express vans have the 6.0 and the 4L80E
Apparently our fleet managers are more concerned with getting the job done 
I agree with you about those engines . . . That old technology works just fine. I also find them to be quite reliable, in fact more reliable, versus the Ford Triton V8 family 
I’ve run into a few valve problems with those engines, but on the whole the LS engines in GM products are simpler, cleaner, easier to maintain, and more of a workhorse than the Ford Tritons.
One exception, the now defunct Ford 6.8 V-10 is a great choice for an RV or a heavy duty truck. They’re smooth and powerful and do great service in those applications.
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Just a code reader won’t help much with no codes. However, there are readers, like Torque Pro, that allow you to log data to help you diagnose a problem. That takes a bit of research on your part to understand what does and doesn’t look right.
You can find lazy O2 sensors that are old enough not work very well but not yet failed. You can find bad coolant and air temp sensors that don’t match what you know to be correct. You can find noisy signals from most anything that won’t set a code but will affect driveability.