Chevrolet Silverado 5.3 V8. My Silverado revs have dropped at tickover so low they , it is hardly registering. As I have to take the truck in for inspection, I need to find the problem. I will be taking the sparkplugs out as my first check.

thank you in advance

I have fitted a new battery, this kept the revs at 500rpm but then faded. The engine revs sound the same, this makes me wonder if the rev counter itself is at fault.

When you have power interruption ie: disconnecting the battery, you have erased the 'learned value' in the engine computer. Now it has to relearn the idle speed control. This could take 50 miles or sometimes a few days of drive cycles.
I would forget spark plugs and look at the induction side of things. The throttle body for most part. One of the most neglected parts of the induction system.
Get yourself some throttle body cleaner, take the intake hose off at the throttle body, and get to scrubbing. I use an old tooth brush and a shop rag.
Don't get too crazy with the spray because you'll cause engine flooding. It should start rough, then have a high idle which will steady out over time.

I have fitted a new battery, so the rev counter has to relearn. Right ho, I will take the truck our for a few sorties. I will also have a look at the intact as you suggest. Ta for the info.