Welcome to the world of emissions equipment. If you are not running the truck hard, you will constantly go into regeneration which is contaminating your oil. This is why you are getting the message to change the oil. The only people reaching 7500 miles are either deleted or they are working the crap outta the truck.
2012 Ram 2500 4x4 ST Crewcab, Cummins 6.7, RCX 5 inch lift, 20x9 KMC XD Spys, 37 inch Nitto Terra Grapplers, H&S XRT Programmer, MBRP exhaust