However,  even Interstate branded batteries have decayed in quality and many produce for them on contractual basis.

When was the last time it was able to start the truck?  Has it been exposed to below-freezing temperatures since then?  If so, highly likely frozen, in which case, do NOT use it- take it in as a core.  Frozen batteries have plates distorted, usually shorted, and fractured busbars.  They become bombs when you attempt to charge and use them.

Pop the caps, and observe the electrolyte level- are they all same?  If not, you have a shorted plate in one cell.  Draw an electrolyte sample with a hydrometer.  If the readings are not all the same, you have a broken busbar in the top of the battery.  If they're are all same, put a charger on it, does it draw current?

If it doesn't draw current, there's a break in the busbars between cells and post.

If it draws current, does that current drop off after a few hours?

If so, charge it up, the disconnect power to the charger, then unclip the leads, walk away for a few hours, come back with the hydrometer and check specific gravity on all cells.

IF you have consistent specific gravity between all cells, put it back in, and give it a try.

Frequently, a perfectly good battery will discharge as a result of dirt on the top of the battery.  It doesn't carry much current, but it'll draw a battery down to flat over a long wait time.  There's probably nothing in your old C10 that uses battery power when off... unless you've fitted it with a stereo, a phone charger, or electronic fuel injection...

The issue is the Lead in them, 85+% is now Recycle Lead from the cells, as with any electrolytic material there is a finite lifespan, much is working well past those limits.  I just replaced the 2017 Exploder battery, Second Time.  Been four in my 2012 Harley.

Nothing wrong with recycled lead that is properly smelted back to it's original form with the impurities removed through the process. Done quite a bit of it myself in younger years but made no attempt to capture, or contain impurities floated off other than a garbage can, and an exhaust fan. 

Not really difficult to remove the harmful impurities through fractionation, but the cost to set up and meet the established standards to operate make it a difficult process to comply with, and be profitable. So much so there are not many options remaining.

 Alternator/reg issues, radio, faulty ignition switch, almost any electrical component can start to fail and cause a slight draw. They can drive a person nuts trying to find. I put a small test light between Pos post and batt cable if light glows a little then start unhooking things 1 at a time until light goes out. IF light don't glow I'll use the voltmeter. 

 I say glow as not be fully lit, that's a slight draw.

you mean... it was alive, and running the truck fine, and now it's dead... or putting a new one in, and it's no different from the old?

If it seems charged by any OTHER test (like a meter directly to the posts), then likely there's a bad cable... which happens... moisture gets in, drawn deep up the wire by capillary action, and turns the copper into fuzzy green powder... which doesn't conduct very well at all...

Charge up and then disconnect ... or open as mentioned and look inside.

You meter should have an  AMPS setting, but probably really low ( a coupe amps)... You can take off the positive cable from the battery and put the meter IN LINE... but dont crank, turn on the light, or anything that pulls AMPS... Just read the value on the meter to see what it is pulling... Probably should be like .02 amps or 20 milliamps, etc.. Not a large value.

  

Use a charger that has an ammeter for a rough indication of charge rate. An aged and completely flat battery may, or may not recover, but this problem is more pronounced with age.

If a battery is extremely low in charge, and upon initial recharge process the current will be very low. This is normal and will start increasing after a bit of time. This could be up to 2 hours before noticeable charge rate change.  

Seen a lot of aftermarket electronic add on equipment direct wired and not ignition switch powered kill, or deplete batteries over the years. 

Worst one I ever worked with automotive related was an early Audiovox remote start system nobody knew was installed onto the vehicle. The receiver portion was buried in the roof and the power wiring was run down the inside of the driver's side "A" pillar and under the dashboard. A little stub wire antenna was affixed to the rear view mirror behind and could not be seen from the interior of the car. 

Owner fought that for quite a spell before dropping it off at my place to finally have it diagnosed and repaired. Took a couple of days as the installation was very good. This would have been highly suspect right out of the gate had the owner known about it but did not, nor had any remotes for it. 

It was removed and problem went away.