slow coolant leak, no external leak shown.? keeping filling coolant once a month
Have you checked the oil/transmission fluid for coolant contamination?
What’s with the question mark . . . ?
Does that mean you’re not sure if there is an external leak?
To be frank, until you put the truck on jack stands and get under there with a decent led flashlight and maybe even a telescoping pocket mirror, I’d say you can’t rule out external leaks just yet. For example, the radiator side tank might be starting to leak, and sometimes you can’t see it until you take a peek into the area covered by the fan shroud . . . that’s where the mirror comes in. Another example are the heater hose couplings, near the firewall. A mirror is often needed to see that they are in fact leaking