[Too Many Projects]

The only issue I ever had with a Silverado heater was on my 2000. I bought it 5 years old and the second winter I had it, it did not blow very warm, even though the temp gauge was at 210. I started reading about this online and people claimed the core plugged with crap from assembly. They, also, complained about the cost to get it replaced and the DIYers said it was a huge job.
As a former mechanic I had run into this years ago with certain models of cars with a heat hose shut off valve and all we did was back flush the hoses, valve and core, so that is what I did and it heated like a boiler after that and never gave another hint of cooling down from debris.

I know this is not the same as your engine not warming up. Maybe you could do an experiment with a piece of cardboard covering about half of the radiator and see if it heats up better. It could be a malfunctioning thermostat that is stuck partway open or opens too early. Have you talked to the dealer about this and is it still under warranty ?