Runs crappy, low mileage, oil in air cleaner

I have a 84 Vandura 3500 made into a small motor home. Right now it runs crappy, very rough, lots of smelly exhaust, has stupidly low mileage (c.6.5 mpg), and oil/gas in the air cleaner. Also, it stalls when coming to a stop after high speed traveling (like the end of an exit ramp on an interstate). It has a rebuilt engine about 4 years old. Mechanics have suggested secondary choke pull-off. What else should I look at and try? Thank you.