I'm not a diesel expert but have had some experience.
Black smoke usually means not enough air or too much fuel.
- Very dirty air filter.
- Check for "critter nesting" somewhere in the air intake.
- If it is turbocharged, the turbo could be frozen.
- One or more fuel injectors are dirty and not closing off - meaning they are still spitting fuel when they shouldn't.
- Weak fuel pump - less likely with black smoke.
Obviously, this is not a complete list and someone may have a better solution for your particular engine setup.
Hope you get the problem resolved quickly and get out there and enjoy your new motorhome.