03 V6 coolant reservoir not draining

Hey Gang! O3 V6, I have a coolant reservoir that’s overflowing and not returning upon cooldown. Weird thing is it’s not overheating.
I’ve read every thread I can find before hitting a wall and finally posting for help.
Replaced the radiator, oem cap, oem thermostat, new hoses. Checked the overflow box and return hoses for clogs or leaks. All signs point to HG pushing gases displacing coolant.
I’ve Done a hydrocarbon test repeatedly and can’t get anything showing head gasket failure. Not leaking coolant, no milky oil or coolant, no smoke Or cylinder codes.
It only does this if I drive long distances, local it works perfectly and purs like a kitten. Only thing I haven’t replaced is the water pump.
I have to turkey baster the coolant out of the reservoir back into the Rad everytime I get gas and that works but I’d rather figure it out! Rig has 316k so there’s that lol. Appreciate any advice if anyone can think of anything I haven’t!
Cheers, Josh

 

Does the hose from the radiator run to the cap on top of the reservoir, not to the bottom?....and there is a hose inside the reservoir that runs from the cap to the bottom? The hose inside attaches firmly to the cap and doesn't have any holes/cracks and it's not plugged?

 

The hose is in good shape, I even put Corbin clamps on all the attachment points to make sure there’s no vacuum leaks. Weird thing is the system works perfectly just driving around locally, it sucks the coolant back into the radiator just as it’s supposed to. Today I got some hydrocarbon tea bag type tests you put in the reservoir, but it’s cold In Montana so I couldn’t even get the coolant hot enough to pour into the overflow. Tomorrow I’ll put some cardboard in front of the radiator to see if I can get the system to expand enough to try the test. Maybe I just have a super small HG leak and it only presents after extended load, cheers