the issue isnt that they disabled their DEF system or deleted their DPF or EGR, it's that they probably installed new ECUs or flashed new fuel maps or something and boosted the power beyond the torque abilities of the transmission attached. In a lot of diesel RVs the transmissions are normally good to 2,000lb-ft, but the engines can be pushed beyond that pretty easily.
My truck owning friends like to blow through transmissions at 50-60K miles by driving their trucks like sports cars and aggressively short shifting while on power. None of them do “truck things” so it’s always a head scratcher as to why they continue to do this.
