Are you burning it?
Cracked head, water jacket?

 

take the front o2 sensor(s) out and look / smell. (like head gasket internally, but getting 100% burned up by the cats...so maybe no exhaust tip coolant stink / vapor yet)

you can add some dye and see if there is a slow leak somewhere,

does your expansion tank get residue like its evaporating, when was the last time you swapped fluid ?
when was the last time you hosed cleaned the outside of the radiator?

if buying things, a new OEM / delco rad cap (s) and OEM / delco thermostat, ...(aftermarkets may not open / flow exactly as they should)

what engine is this ? Miles on this?

 

It’s a 5.3 with 190k on it I just got the truck a year ago and have been plugging away on things as I’ve noticed some before I could drive it. Started with the brake lines from there. I’ve done Tyrod ends and outer Iler Pittman arm bracket upper control arms. I had to do new bushings and body mount bushings, and then doing that found it. The body mounts were pretty compromised or welded in plates on those the frame overall is pretty solid, but I’ve had a recent front differential driver side stub shaft slides in and out what I would assume is about an inch of In-N-Out play the radiator fluid and I’m guessing it’s a oil pan gasket issue I don’t know the maintenance history really on the truck so again as things arise I’m taking care of them as for flushing the radiator or clearing it in general I have yet to do it at all. I will, though under your recommendation I’m not a mechanic mechanically inclined, but I don’t know really what I should or shouldn’t be doing I guess in troubleshooting some of these issues.

 

Sounds like you have a head gasket issue and you’re burning coolant.

 

I had a different brand of truck that kept losing coolant (at a larger rate than you). No leaks, couldn't figure out where it was going. I did send out a couple oil samples to Blackstone and they confirmed high levels of coolant in my oil. This eventually gave me some more ammo to get approval for a new engine under warranty.