I think a major consideration should be availability of service. With my Cummins ISX15 and Freightliner chassis, I can get service just about anywhere big trucks get service. Just took a 6,000 mile road trip, encountered a dripping heater core along the way. Was able to get it serviced at a Loves Truck stop where they did a temporary bypass of the core and I was on my way in less than an hour for $50. No issues or hesitation by the mechanic of it being an RV. I asked him about parts availability and he said Cummins are the easiest by far. The Detroits aren't too bad but are suffering a bit by parts supply issues but are not too difficult to work on. Not sure about the Merc motors, especially if you are considering an older rig.
2014 Renegade Classic, 45', Freightliner Cascadia 126
Cummins ISX15 with 13 speed Eaton auto shift
1999 Jeep Wrangler toad