Fuel/DEF fluid cost depends on how much your drive.
Oil changes can be pricey about $125 if you do it yourself and use correct spec'd MB oil.
Your biggest issue and cost down the road will be unscheduled maintenance from the modern emissions system on the chassis. This can cost you time when under warranty and a lot of coin when out of warranty.
is scary reading.
While I don't own a MB powered RV, I have owned an SUV powered by a MB OM642 3.0L turbo diesel for the past 11 years. This year, we have only put in $4000 in repairs-a new wiring harness and ECU. I normally do my own work but I couldn't afford to spend 15-20 hours putting in the harness myself.
Good luck in your decision but i would personally never buy a MB diesel powered RV because they typically lack cargo carrying and/or meaninful tow capacity. A small Class C I think is too much for a 3.0L diesel.