I'm ruling out the thermostat because you say you're not overheating
. If the thermostat was failing to open your temp gauge would spike immediately.
Boiling point of a liquid depends on lots of things, but one of those things is pressure. More pressure = higher boiling point. Your coolant system pressurizes itself as it heats up, and that pressure lets the coolant get to higher temps without boiling. I think you're boiling because you don't have enough pressure. That's normally a bad radiator cap, if it's not, it could be a bad hose, it
could be a blown head gasket. Normally I'd ask if the truck was burning coolant, but if you're losing coolant out the overflow bottle on a regular basis then you wouldn't be able to tell anyway. You could still inspect your coolant hoses to see if there's any crusty pink or light green residue, that's a sure sign of a leaking hose and that would cause your loss of pressure. But I think it's a radiator cap because by your description, the truck doesn't do the boiling at lower altitude, only when you get home where the air pressure is less. Possibly just enough less to make the radiator cap misbehave.
Rad cap is cheap. If you replace the cap and it still does it, get a block test from NAPA or wherever and test your coolant for exhaust particles. If the block test is negative and the cap is good have a rad shop do a full system pressure check to find where it's losing pressure at (I don't know how far away you are but I like JMAC on State Street in SLC for radiator stuff).
If it's not a pressure issue at all then it's over my head, but that's my bet.
