Having you tried driving it with the trans in manual low? Being that it is a 12v, you don't have a computer triggering the check engine light. So odds are you have a low coolant sensor or oil psi sensor unplugged.
12v cummins isn't a heui engine. Should have either a p7100 or possibly a ve44. Both are mechanical injection.
Vacuum leak wouldn't cause power loss. If timing was off you wouldn't have power in either direction. How much of a hill are you on?
If all the engine stuff was done right, I'm leaning towards a trans or brake issue. Shift it into manual low (position labeled 1 on gear indicator) and see what you have. Also check trans fluid because that being low can keep the trans from acting right.