I have had two Reduced power / Check engine episodes so far on this new truck. Both times were pulling 3000# travel trailer, in tow mode, on 6% grade, in cruse control at about 65 miles per hour. Limped into dealer. Three days of diagnostic only to say "Soft failure"
I believe it is an issue of towing in cruse control as I no longer tow in cruse and I no longer have Reduced Power issues. But from looking over many threads, Reduced Power is a common issue
We can empathize, sincerely. We can all imagine your frustration with this, whether it has happened to us or not. As one more data point, never happened to us with over 15K miles of towing, all harder than what you describe. We HAVE lost our exhaust temp sensor (not the proper name for it), which lost us TT mode, which lost us 3 days to get the part and get it fixed on warranty. But we just camped, drank, swam, which made it part of the adventure.
I don't think it was the transmission. Even if it was smokin' hot, you didn't work your truck that hard. Especially if you were in TT mode. Of course I'll stand corrected if you come back to tell us that you saw trans temps above ~230F. I don't think it was your cruise control. Why would it be? I DO think that your dealer diagnostic is weak. Is there really a code called "soft failure"? Not rhetorical folks, I'm aksin. Lots of you know these codes. I don't. But I can't imagine any reduced power event not being traceable to an actual operational condition that you could avoid or remedy....
Oliveman, since you didn't mention a trans temp, I'm thinking that you're not periodically clicking thru your info buttons when you tow heavy. If not, it's a good habit to pick up...