Hello, not knowing history of truck this may be a little long and comprehensive. I have to ask but have you checked hoses at all connections, water will travel from anywhere, everywhere. Heater hoses, clamps tight, check, affirm. Heater hoses, heatercore. The new heater cores are so cheap that a little pressure and leaks become streams.
cucvrus has givin good advise about the rear cylinder head coolant plates, they corrode, leak at the bolts, and loosen from heat cycles. Check and affirm that the bolts are snug, tight. I use coppercoat, or Teflon paste on the bolt or stud threads and silicone around bolts going through the hole to keep from leaking.
Wipe down engine as much as possible at the rear and it may take many times to look at before you can localize and pin down the leak. Day or night, flashlight, real bright light so that you can see everything. You may even take off hood to get a better view from topside. Put heavy towels or rags, or small squares of plywood at the hood corners to keep from cracking glass. You may be the best at removal but it isn't worth the chance.
Do you check antifreeze for concentration? Was the antifreeze good for the lowest freeze point in your area, and below? I ask because one 6.2 that I replaced the owner could not find leak and I took off starter and block had horizontal crack between freeze plugs. Last hurrah block seal, but is it worth the chance?
I hope it is just the coolant plate or heater hose leaking at the clamp. Sometimes it is that simple.
