2015 F250 (17,000miles) traveling in west at high altitudes pulling a fifth wheel and have had four different "check engine lights" serviced at three different Ford dealers in a 10 day period. Fixes include 3 manual regens, replace NOX sensor, replace entire exhaust filter unit, replace EGR. Getting ready to start on next leg of trip and concerned about check engine and limp mode while pulling a 10,000 pound trailer. I have advised all three Ford service reps that the truck has not used any Def in over 6,000 miles of pulling trailer. All reps did not think that fact was significant and proceeded to simply replace parts as indicated by diagnostics. Last dealer even used Ford engineers "hot line" to diagnose. Result was replacement of EGR valve. Tried to add def this afternoon and would only take a few ounces. If the def injector pump has failed or injection nozzles plugged shouldn't I get some type of fault code? Any suggestions?