I have an 06 6.0 with a 03 6.0 swap and i have put new aluminum radiator,egr delete with egr still in ,new turbo, studded,head gaskets, running sct x4, 4" mbrp exhaust, i have tried diagnosing everything possible to find out why my coolant is dissapearing ,i havnt checked the thermostat Or oil cooler cuz well i havnt had time but thermostat could be an issue and the oil cooler is new from sinister but it was running fine untill i hooked a trailer too it and now even without a trailer coolant disappears and my truck runs at 190 to 202 at idle or running in town and not even getting on it more like 30mph everywere i go

 

Tuner is set to performance i have ordered a low temp thermostat to see if it works head gasket and all work was recently done about a month or 2 ago, no white smoke from exhaust and it doesnt puke all tho it did push back some coolant when i opened it yesterday like it had too much it didnt spit it just overflowed realy slow and it didnt boil over or sound erratic when it happened i have had a blown headgasket before and the resevoir literally sounded like a volcano every time i tried pouring coolant or if i opened the cap

 

I would start with replacing the cap to the degas tank. They are supposed to hold a certain pressure so the coolant system works properly. If the cap is weak and doesn't keep the coolant system pressurized, the coolant could theoretically boil off causing your level to drop. Just an idea, but you want to make sure to get the newer updated cap. Also, make sure the coolant level is at the minimum line. The specs changed and the minimum line is actually the level your coolant should be at when the engine is cold.

 

Keep in mind, the higher temps will be seen during low speed driving when you aren't towing. I have always noticed with all my vehicles that my coolant temps are lower on the highway/freeway than driving around town. I attribute this to less airflow through the radiator. I also live where ambient temps are over 100F more often than not during summer, so that may play a role. Ambient temps can get over 117F, without factoring in the fact that asphalt temps are even hotter, resulting in A LOT of vehicles overheating that wouldn't normally overheat.
It was about 96F here today, very sunny, and my 200F coolant temp driving around town didn't even raise an eyebrow. climate plays a huge role is what I am getting at here.

definitely replace that coolant (degas tank) cap if it has been awhile.

The OEM radiator has been known to leak, so it wouldn't be a bad idea to look it over.
Also, like billbot said, the coolant level in the degas bottle should be less than you would think, especially with the factory fill level markings.

If your coolant lines are starting to dry-rot, (once again, climate issue) it wouldn't be a bad idea to give them a once-over and make sure none of them are leaking.
how much coolant are you losing? my coolant was overfilled and lost a bit under hard driving or towing, until it leveled a little over the factory cold fill mark when hot. I am also studded with a bulletproof EGR that is essentially disabled through tuning,

 

I lost quite a bit of coolant i added about a gallon before i left on my trip and its all gone when i arrived i only driven it maybe about 2 hours and my temp was about 202 to 205 the whole time with empty trailer

 

Put a pressure gauge in the coolant return line. If pressure increases rapidly, could be a blown gasket again. Should be less than 16 lbs as that's the cap limit.

 

Another note is when i would take off from the light the temp would some times just down 202 to 188 sometimes to 175

 

And if i leave it running in the driveway it stays at 175

 

Sounds like your thermostat might also need to be replaced. Also, measure specific gravity of the coolent mixture, should be atleast 45% or even better 50% Ford Gold or Xerox G05 gold. Anything less will not hold your boiling point high enough. The other thing to check into is water pump flow. Just suggestions, to look into but I think your thermostat might be the first thing to address.

 

A gallon of coolant seems like a lot. How is your egr system deleted?

 

This COULD be an indication the coolant level has dropped below the level of the sensor or an air pocket is directly below the sensor. When the business end of the sensor isn't in direct contact with liquid coolant the temp can drop and fool you into thinking everything is ok when in fact it is overheating.