Sorry you had this happen, but I'd say with 99% certainty this was due to a regen. Usually you won't get a notice that a regen is taking place other than lower MPGs, but keep in mind that your exhaust temp can get up to 1,300 degrees during a regen cycle. I'd suggest any of three options; buy a 5" 45 degree angle tip and bolt it on to the outlet when you tow, cut the outlet a little shorter and permanently mount a 45 tip on the end, or buy a DPF-back exhaust that comes out the side. If it happened once, it will happen again unless you point the exhaust away from the trailer. I pull a travel trailer and have some extra room so the stock angle is fine, but your fifth wheel is closer to that high heat.