evernlc

Okay so my truck is a 2014 ram cummins 3500. Ever since i bought the truck i have had coolant problems. When new i would take it to the dealer but they never found anything wrong with it. Then i just stopped taking the truck back since i use my truck for work and I'm away from home. When new i would add coolant every once a couple of weeks. Now i top it off and my deam light comes up low coolant like 20 min after i fill it up. I can't find no leaks at all. I have put 18lbs of pressure on the cap to check for leaks and nothing. I have bought the dye they sell and check it at night and i can't find anything. There is no white smoke in the mornings either. There is no coolant in the oil. I did the egr delete just incase but no luck. I really have no idea i just keep my light of low coolant on all the time. I have thought about the coolant sensor but why do i have to keep adding all the time? And then sometimes it over heats when im driving and i bought a new thermostat from the dealer and i also did my water pump recall and nothing changes in this truck. Anyone has had similar problems like this before?