I don't have a MH but do have an 03 GMC 4500 Duramax/Allison 1000 and a couple other GM Duramax/Allison trucks at work. On the 4500/5500 chassis there is a main harness leading from the ECM/TCM down to the trans. Goes from the passenger side firewall down past the removable fender liner and on down to the bellhousing. GM had a known abrasion issue with that section of the harness. I've repaired it on ours once myself and my dealer has done it another 2 times. All 3 times it has caused shifting/no shift issues and multiple trans codes. Another very common failure point is the park/neutral switch mounted directly on the trans at the shifter pin. It's a common issue on all GM automatics largely due to poor sealing and proximity to road spray. When it fails it can also result in abnormal or no shift issues and is typically accompanied by a malfunctioning "PRNDL" display on the dash.
In general these were pretty poorly built cabs by GM. The chassis, engine, transmission, axles, etc. are stout but the cabs and associated harnesses/parts hailed from the dark days of GM. We are moving away from all the 4500-8500 series chassis we have in favor of Ford and International because the GM stuff just doesn't hold up, at least in commercial use.
Rob, Sarah, Ella, Ava
2017 Eagle HT 29.5BHDS -- 2011 F350 CCSB 4x4 PSD