I would start by doing a combustion gas leak test on the radiator. https://www.amazon.com/Lisle-75500-C...0220526&sr=8-2
This is the one I use, reasonably priced and works well.
I think you're ultimately going to be looking at a head gasket or cracked head/block. Sludge in the coolant is usually brown from iron oxidizing as the coolant's corrosion inhibitors wear out. I suspect the grey sludge is a head gasket repair in a can. There's a decent chance someone dumped a can in and sold the truck immediately there after.
The combustion gas leak tester will save a lot of time poking and prodding around trying to figure out what the culprit is. If you can see bubbles in the radiator cap shortly after starting before the coolant has warmed up, you can pretty much guarantee it's a head gasket or crack.