Problem with idle run ruff rich smell . gas milage is good 13 mile to the gallon last tank.

You'll need to know how big the fuel tank is on your truck. They used different sized tanks depending on the box or body fitted. I think the standard body box was a 26 gallon tank and the long body box was a 34 gallon. This will help you work out your actual miles per gallon.
That model's factory figures have it at between 12 to 13 mpg average. How and where you drive your truck can also affect you fuel consumption quite a lot, City driving, Hard accelerating, Lots of steep hills where you drive and Driving Off Road all can ruin you MPG figures. Using the your numbers that does seem a little lower than it should be. When was it last given a full service? Have you changed fuel supplier, (different gas station). All of these affect MPG Do you have any warning lights on the Dashboard? Use the comment button below with more information and I'll try to help you more.

Using your numbers here's the math.
26 gallon tank - half tank = 13 gallons. 34 gallon tank - half tank = 17 gallons. You got 104 miles.
Sum used is 'Miles achieved divided by the number of gallons used'.
(26 tank) 104 divided by 13 = 8 Mpg.
(34 tank) 104 divided by 17 = 6 Mpg.

If the engine is running rich, you should have a check engine light on.
If you do, have the computer scanned for trouble codes.
It could be a fuel leak in the line or around the injectors too.

check airfilter

check for a leaking fuel rail,or injector,or the fuel pressure regulator,which is a vacuum operated valve at the end of the fuel rail.the high idle is caused by a vacuum leak,and if the regulator is leaking,that could be the fuel smell

Check the fuel filter, it might be clogged.

this is what I would do. I have the 1993 model and get 22.5 to 23 miles per gallon.
I would change the plugs, change the plug wires, change the fuel filter, change the air filter.
if you have done this lately then one overlooked item would be the coolant thermostat.

when you first crank the car and the engine is not warmed up, it runs in a "closed loop" mode and it gets bad mileage. the computer doesnt care about mileage at that point, it just wants it to run good.
after the coolant gets up to temperature, there is a coolant temperature sensor ( should be located screwed into radiator somewhere) that alerts the computer it is up to operating temperature. the computer then goes into a "open loop" mode and the computer then constantly changes the air fuel mixture to make the best out of emissions ,milage, and power,...
so, it it is not getting up to temperature it could be the coolant thermostat is stuck open. if you cant remember when the last time its been replaced then you may want to go ahead and do it.
if that dont work, it could be the coolant temperature sensor itself.
When I was carpooling in my caddi I drove 7 miles down the road to catch my ride. I get 15 miles to the gallon. and spend more on gas then if I just drove by myself. when I drive to work my self (35 miles) I get 23 miles to the gallon with stop and go averaging speed of 50

I have the same issue of low gas mileage. My 2008 Kia Rondo with V6 (2.7Lt) gets 20-21 mpg in average. Even for highway mileage, it is around 21-22mpg. Very poor gas mileage. Disappointed. I hope it would improve in the future.

Wide tires make less gas millage, get narrow wheels, let the car accomplish speed slowly from 0 to 30, look at air filter,if shortly replaced, oxygen censor, 15 miles is normal 16 miles per gallon is good for a V8

clean out the carb and fit an inline fuel filter between the pump and the carb .