02 overheating

[mike_b]

I have an 02 F-250 with 185K miles, that is overheating when pulling a 5th wheel RV ( 11K lbs ) on an incline. I can pull it all day on a flat road at 65 mph, but as soon as I start up any an incline the temp gauge will go to 75% and if I don't slow down quick it will go the 100% and the check gauge light will come on. After topping the incline and start to decline the gauge will go right back the normal range. I have changed water pump, thermostat, both hoses, temp sensor on water pump, fan clutch, and had the radiator and engine flushed. I don't think the engine is actually getting hot, because I have not had it to boil over or lose any coolant. I have had the transmission flushed and filter changed and the gauge will stay in the normal operating range. The oil and filter have about 2000 miles on them and that gauge stays in the normal range. This first happened in Missouri on my way back to Texas in July, and then I made the changes to the cooling system and thought I had the problem fixed. I made it almost to Denver and it overheated again. I would really like to find a fix before pulling it back to Texas in a week. Any suggestions would be greatly appreciated. Thanks in advance.