I took my 27' ultra light 5th wheel for a 3 hour drive to go camping today and once I hit the free way doing average 100 kmh my egt were around 1100 on average. It would spike to about 1200 or more on hills. Then I would let off. I kept going keeping egt at 1000 and above because that's the coolest it would run on hwy. I stopped and pulled over once the low coolant light came on. Worried as hell I checked under the hood and resevoir was near empty. Looking for where it went I noticed the whole passenger side of truck was covered in coolant along with trailer. I pulled inner fender to check rad hoses and engine. But no leak. Finally I think I figured that it came from coolant reservoir overflow tube. So was my coolant so hot that it was boiling? I noticed moisture build up on inside of coolant reservoir. Help me guys I'm sitting at a camp ground frustrated and wondering what to do.