2015 F350 Exh Overload Drive to Clean

Have a new 2015 F350 6.7. Currently at 7,000 miles. Have observed the message "Cleaning Exhaust Filter" and did my research and read manual and understand what's happening. Today, on I-80 running at 65 mph got a message of EXH OVERLOADED DRIVE TO CLEAN. Since I was running along at 65 and had been for several hours kept going to get it to clean. The manual says you must drive it over 30. After about five minutes and six or so miles got message EXH OVER LIMIT SERVICE NOW. After a few miles got the message CLEANING EXHAUST FILTER. Then it cycled back to the OVERLOAD message and I reduced the speed to 60 and kept driving. Then it cycled on toe the EXH OVER LIMIT message. Cycled through all these four or five times. Called a dealer and stopped in Rock Springs, WY today (7 Aug 14). They tell me they must do a manual regeneration (the cleaning process) and that Ford will not cover this and I have to pay the $118 service fee. The truck was serviced at 5,000 miles. Why would this happen? I do not believe I have had excessive idling or short trips. Of the 7,000 miles about 1,000 is on the open road at speeds of 70 to 75, about 5,500 is on the road pulling a 12,000 pound 5th wheel load at 60 to 65 and about 500 is mixed in all that on local trips. The service manager and two of the mechanics sound good at their jobs, knowledgeable and helpful. They say one thing may be using a cheap auto parts DEF rather than the Ford brand DEF. Again, has anyone experienced this?