Chevrolet Repair: 84 winnebago Rough idle only at start, gas efficiency, manifold vacuum

Question
The winnebago is on a chevy P30 chassi with a 454ci 7.4l engine. I had no prior issues with the RV except for exhaust leak. I just replaced my exhaust front to back including manifolds and Air injection tubes. The mechanic also replaced spark plugs since he had to pull old one to remove exhaust manifold. The next day when I started the engine it ideal extremely rough, darker smoke from tail pip,the gas efficiency gauge dropped to the red,and it felt like it wanted to die. after giving it a little gas for approx. 30sec or so (while idling rough)it then began to ideal smooth again and gas efficiency meter returned to green. It has continued to do this now for a week exact same issue at cold start. After engine is warm it starts like it did prior to the new exhaust quick and smoothly. What may be the problem and how to check and fix? Thank You

Answer
In the extensive repair, a vacuum line is probably not reconnected or broken.  The cold start is bad but the computer adjusts for this condition once in cold loop.  Take it back or look for the problem yourself.

My best guess.  Verify with a vacuum guage.  If low manifold vacuum, then getting vacuum elsewhere.