F350 Exhaust Filter Problem

[Chaseurs]

Hey Gang
I've got a 2015 F350 with 6.7 diesel. 40,000 miles. Pull 2011 3000RK
On our recent trip from Idaho to Arizona we drove 200 miles and stopped for a stretch. Went another 192 miles and filled with fuel and def. After an additional 72 miles (464 miles from home) the "Entering low power mode" notice came up and we made it to a wide spot at 15 mph and pulled over. Shut truck off for 10 minutes. Re-started and the next message was "drive to clean exhaust filter." Normal driving again.
Stopped for the night after a total of 530 miles.
Next day drove 80 miles and filled with fuel and topped off def tank.
Drove 122 miles and truck went into "low power mode again. Pulled over, stretched, and resumed to same message " drive to clean exhaust filter". Normal driving (I never exceed 65 mph). After 66 miles the cleaning exhaust filter notice came on. Normal driving and arrived at our winter home after 925 miles.
This summer I drove approximately 2500 miles to New Mexico and back without trailer and never an issue. Ford dealer here in Phoenix is worthless.
Driving 15 mph on a crowded road with no where to pull off is not much fun. Any suggestions?
Thanks
Doug & Kathy